Classes
Five classes stand out to me as ideal choices for entry to warshaper.
Pugilist Fighter
Pugilist is a variant fighter from Dragon vol. 310. It gains Improved Unarmed Strike, slightly increased unarmed strike damage (but it doesn’t scale like the monk’s), the Endurance feat, and then one other feat (or special “Pugilist ability”) of your choice from a decent-size list (comparable to the size of the core fighter bonus feat list, but much smaller when you consider all of the supplement feats that a fighter can usually take). It’s kind of solid for all the feats you get in one level, but I’d definitely not take more than the one.
Pugilists give up martial weapon proficiency, but retain all armor proficiencies.
Knight
Knight gets d12 HD, heavy armor, and Bulwark of Defense and Test of Mettle in the first four levels. You also get the Fighting Challenge, which is OK, and Mounted Combat, which... is maybe useful? Anyway, the first four levels are pretty good (in fact, it is often claimed that knight has five good levels: 1, 2, 3, 4, and 20).
I would only go with knight if I got at least three levels; Bulwark of Defense is a really nice feature. I’d want Test of Mettle if my Charisma was good, but a level of pugilist or crusader could fit in well.
The big problem with knight is the Knight’s Code, which is possibly even stricter than the paladin’s (though the penalty is less severe). It also demands that you remain lawful: that makes it hard to also take a level of barbarian, and you want a level of barbarian, because...
Barbarian
Barbarians can get Improved Unarmed Strike as a bonus feat, but it’s not worth it to do so. Instead, you want a level of barbarian for relevant stat bonuses, d12 HD, and most importantly, the lion spiritual totem from Complete Champion. Pounce gives you absolutely critical mobility.
Warblade
Warblade gets medium armor proficiency, not heavy, but also gets d12 HD, and a number of really useful maneuvers. Tiger Claw can let you attack more often, which is nice, with wolf fang strike and sudden leap.
Crusader
Doesn’t get Tiger Claw like warblade does, and only a d10 HD, but it does get heavy armor proficiency, and more importantly, Devoted Spirit is solid, reliable non-magical healing. Crusaders are the best in-combat healers in the game, while remaining solid martial forces in their own right. Plus Steely Resolve can dramatically improve your ability to remain standing.
Crusader 4 itself could be a solid entry to warshaper, though I’d probably go with levels of barbarian and pugilist first.
Feats
Superior Unarmed Strike
This feat from Tome of Battle gives you a scaling unarmed strike damage similar to (slower than) the monk, if you care. It’s decent enough, particularly when you’re getting size bonuses.
Improved Natural Attack (unarmed strike)
It’s a damage boost that stacks with everything; might as well.
Note that in theory you could replace this with a fanged ring if you were in a game where you could expect to get a 10,000 gp magic item any time soon, but you aren’t
Stone Power
Another feat from Tome of Battle, this allows you to take attack penalties (similar to Combat Expertise, no larger than your BAB or 5, whichever is less), gaining a number of temporary hit points equal to twice the penalty. The temporary hit points last 1 round, but you can continuously add up to 10 HP that basically “doesn’t count” as damage you take ever round. At low levels, that removes a lot of damage.
Suggestion
Barbarian 1/Pugilist 1/Crusader 2
Heavy armor, a d12 HD and three d10s. Rage and pounce from barbarian, Improved Unarmed Strike, Endurance, and another feat of choice from pugilist, Steely Resolve, Furious Counterstrike, and Indomitable Soul, along with a bunch of maneuvers, from crusader.
For maneuvers, you get five 1st-level maneuvers, a 1st-level stance, and one 2nd-level maneuver. The crusader awkwardly only has only six 1st-level maneuvers to pick from, and leading the attack and vanguard strike are very-nearly identical anyway, so your choices in 1st-level maneuvers don’t matter that much. Anyway, vanguard strike is slightly better than leading the attack, and you don’t need both, so I’ leave out leading the attack and get the other five. For stance, martial spirit and iron guard’s glare pretty much are your best options; the former gives you more healing, while the latter draws attacks towards you by improving allies’ AC without improving yours. The 2nd-level maneuver basically should be mountain hammer, hands down.
For feats, Improved Natural Attack (unarmed strike) and Stone Power are my picks. From pugilist, probably Improved Initiative, since the list is pretty weak; Two-Weapon Fighting or Combat Expertise might be worthwhile if you want to go the TWF route or the tripping route. I’d probably get Superior Unarmed Strike at 6th (unless going for Improved Trip), and Snap Kick (also Tome of Battle) at 9th.
Best Answer
Level 1
I think you’re better off taking your first level in Fighter, or an alternative, than Artificer. It’s a difference between more HP and more skills, but I don’t think you’re going to use a lot of skills beyond Craft and Use Magic Device. But I’d probably rather something better than Fighter...
Problems with Fighter
I don’t think Fighter offers very much here. The class is generally quite poor, and only a good idea if there are a series of specific feats you have in mind that you cannot get any other way.
Note that tanking in 3.5 is not easy; the system does not offer very many ways to do it effectively. You cannot simply be capable of taking a lot of damage, you also have to be able to handle a wide variety of magical effects so you cannot get sidelined, and you need to be a real threat in your own right so you don’t get ignored.
Fighter can offer these things with very careful feat selection, but there are better ways. You don’t need proficiency in any particular armor, you’re not using Tower Shields, and Base Attack Bonus and large HD are available on other, better classes. The Fighter’s notoriously weak Will save (that 12 is quite likely to go into Wisdom) mean that spellcasters may be able to very trivially prevent you from protecting anyone.
If you insist on Fighter, though, you can do a lot to salvage the class by using the Dungeoncrasher alternate class feature in Dungeonscape. Get Improved Bull Rush, Shock Trooper (Complete Warrior), and go to town on enemies. If you go this route, you’ll want exactly 6 levels of Fighter. At which point, you’ll qualify for Warforged Juggernaut, which I strongly recommend here.
Alternative Martial Base Classes
Just from Core classes, Barbarian is a very good choice. You cannot use Infusions while Raging, but most melee Artificers are about long-term buffs, which don’t go away when you Rage. If you have Complete Champion, you can get Pounce instead of Fast Movement: getting the ability to move and full-attack is paramount to your ability to actually be threat enough to draw enemies’ fire away from your allies.
Of course, as is usually the case in these discussions, Crusader and Warblade from Tome of Battle are far superior choices. The Crusader, in particular, gets some of the very-few true aggro-controlling abilities in the game.
For completeness, I’ll also point out the Knight from Player’s Handbook II. It’s unfortunately rather underwhelming, but Test of Mettle is one of the other very few aggro-controlling abilities in 3.5. It’s got a lot of limitations, lots of things are immune to it, and the DC depends on Knight levels (of which, most are bad) and Charisma (which Warforged take a penalty to), so I don’t really recommend Knight.
Recommended Prestige Classes
The Warforged Juggernaut from Eberron Campaign Setting is an excellent choice for prestige class, regardless of how you start. The immunities it gives you are great for not getting sidelined too easily. It’s prerequisites are also ideal for a Dungeoncrasher; a Dungeoncrasher Fighter 6/Warforged Juggernaut 10 is a pretty solid, simple build. I wouldn’t bother with Artificer as a Dungeoncrasher, though; you won’t be able to fit in enough levels to make it worth it.
If you do go with a Barbarian/Artificer, you may find Rage interfering with your infusions/magic items. There is no official way around this, but you might ask your DM if you could adapt Rage Mage (Complete Warrior) to allow you to enter with 2nd-level Infusions instead of 2nd-level Spells, and have Spell Rage apply to infusions and magic items instead of spells. It’s a pretty minor change, and Rage Mage is hardly an amazing class, so it might fly. Personally, though, I think it’s unnecessary; simply being careful about when you start to Rage should be enough. Combat Casting is a pretty obnoxious feat tax.
Something Different: Straight Artificer
I’d also seriously consider straight Artificer, since the Warforged substitution levels in Races of Eberron are quite good. Your base HP will be low (but you should have high Constitution), and ¾ BAB will hurt (but probably not as much as you think), but personally I’d want to get Artificer 5 sooner rather than later. Artificers also get pretty solid tanking abilities, since they can get things like wand of shield other, and various survival spells and infusions. Magic items can go a very long way to shoring up the Artificer’s weaknesses (actually, at high levels of optimization, the Artificer is one of the strongest classes in the game, and capable of utterly devastating any of the other classes I’ve mentioned in this answer; that’s non-trivial to accomplish though).
Personal Suggestion
I'd probably dip Barbarian, take Artificer long enough to get some useful abilities, and then go with Warforged Juggernaut. You don’t quite qualify for Juggernaut as a Barbarian 1/Artificer 5, so a dip into Crusader for some maneuvers would be a good idea. Alternatively, I might drop that level into Rage Mage if I could convince my DM to allow the adaptation, but only if I really felt the like Rage was interfering too much; I don’t think it would really.
So I’d plan on something like Barbarian 1/Artificer 5/Crusader 1/Warforged Juggernaut 10 or Barbarian 1/Artificer 5/Rage Mage 1/Warforged Juggernaut 10.
If Fighter needs to be a part, I’d go Fighter 6 with Dungeoncrasher, and never look back: I’d enter Warforged Juggernaut at that point, and if I finished it, I’d find something else to do. Without Dungeoncrasher, I think Fighter levels are largely a waste of time.