If you are looking for out-of-game information that you can use to help you catch up, I suggest sites such as WoW Insider and MMO-Champion.
WoW Insider is a blog oriented towards casual players and has pretty good guides for all classes and specs.
MMO-Champion has a billboard format that presents the latest news pertaining to World of Warcraft, focusing on end-game, future patches and expansions.
The website Icy Veins has very good guides oriented to the hardcore player and deals with a lot of theorycrafting.
Max DPS is a great site that parses your character's armory and outputs a list of armor and weapons that would potentially increase your healing, tanking or damage performance in raids. Information on where to get such items is also provided.
Lastly, the World of Warcraft website and it's forums are a great source of information.
For solutions in-game, asking other people is always the best way to catch up on what you should be doing. Even if you are trying to avoid this, I would strongly suggest looking out for a guild with a good reputation and helpful players. These guilds are definitely rare, but trust me, they are out there.
Alternatively, the trade chat in main cities is filled with people looking for other players to engage in PVE and PVP combat. Just peeking into this channel will give you a good idea of what content are people tackling. A lot of acronyms are used when referring to instances and battlegrounds, so I suggest you check this page if you find yourself lost.
The Dungeon Journal is one of WoW's latests features and it can help you figure out what is the PVE content pertaining to the current expansion.
Billboards in main cities are meant to guide players into zones they should be leveling at or introduce high-level players to quest chains that might be of their interest.
The Dungeon, Raid and PVP matchmaking systems are also great ways to regain familiarity with the game. Looking for Raid and Looking for Dungeon enforce an item level cap that you must achieve before being able to queue. Be adviced that people you join will probably expect you to know what you are doing in certain encounters.
I use Pawn for this.
Pawn gives every equippable item a score based on stat weights for your spec (you can also specify your own weights, determined by tools like SimulationCraft). When you compare two items, it will annotate the score of the better item:
But, by default, it will compare the combination of main hand and off hand to any two-handed weapon, so even if a dagger is worse than the staff, Pawn will mark it as better as long as the off hand item makes up the difference.
Best Answer
That bar you are seeing has nothing to do with a cap. That is the artifact power required to unlock the next trait, nothing more. It's mainly there to just remind you to go unlock a new trait when you have enough.
By the time you finish your artifact, with all 34 traits and all 20 of the final dump trait you will have spent 65,256,330 Artifact Power. It's impossible to waste AP, so don't worry and just keep collecting.