The system is designed to accomodate this
...but without the DMG it's a little tricky.
The science:
Basically, each enemy has an XP value. This is how much XP it's worth when it's defeated (divided among those who defeat it), but it's also useful for building encounters.
Here's how you build an encounter in 4e: You take the XP value of a "standard"-type enemy of the same level as the party, and multiply that number by the number of PCs in the party. The result gives you a "budget" that you use to "buy" enemies to create an encounter of average difficulty (the party is unlikely to die, but will expend a noticeable amount of resources --consumables, healing surges, daily powers-- during the fight).
For a more difficult fight, increase the level of the standard-type enemy whose XP you're using as the baseline multiplier to get your budget, up to four or five levels above the party. For an easier fight, drop the level down by three or four. The extreme ends of this will produce boss-level fights, or make-the-players-feel-invincible routs.
The art:
The DMG recommends actually using enemies up to five levels higher than the party for boss fights, but in my experience this is more frustrating than interesting; it's better to use "solo" type monsters of the party's level. The challenge level will be similar but more fun.
Combine standards, elites, and minions for interesting fights. Minions die quicker and elites last longer, so if there's an NPC or ability you want to be present throughout the fight make it a tougher monster type.
Use soldier (defender) and lurker types for longer more drawn-out battles, brutes and strikers for shorter, more intense fights.
If you've got a combination of enemies whose abilities support each other in significant ways, or you're adding strange terrain, remember that this may make the fight harder than its XP budget will imply.
The mechanics of monsters changed partway through 4e's tenure
With the publication of the Monster Manual 3, monsters got their hp reduced, their damage increased, and their powers were made a bit more interesting. This makes fights take a little less time while being a little more tense, but the overall resource drain per fight is pretty much the same. If you can get your hands on them, use post-MM3 monsters whenever possible until you're familiar enough with them to adjust the earlier monsters to fit that ethos. If you can't, don't worry about it too much. You'll learn to fiddle with monsters based on experience, and until then the fights will be a little more tedious than they'd be with MM3 monsters.
To that end, seriously consider a D&D Insider subscription. It provides a searchable compendium of every mechanic --rule, monster, item, class, race, etc-- ever officially published, a solid character builder AND a solid monster builder, and downloadable access to all the Dungeon and Dragon magazines for 4e. AND all the errata are kept up-to-date across the compendium and builders. I was suspicious of the service at first, but quickly found it to be nearly indispensable.
The biggest key to creating interesting boss fights (in my experience) is to introduce an element of surprise or guess work. Fights are boring if they're just constant dice rolls back and forth where everything goes as expected. But you can make mechanics which keep the players guessing and on their toes, which force them to constantly be thinking about what's going on or what to do next.
For example, in the culmination to one of my campaigns, I had the final boss have a "shifting immunity". Each round, I would roll on a table to see what type of energy or effect he was immune to that round, and each immunity had an associated color. So each round, the boss would change colors and have different things it was vulnerable to.
This forced the players to be paying attention and didn't allow them to just sit back and spam the same moves over and over. ("What? That fireball worked last time...") It also made it interesting for them, because, in addition to being a shifting immunity, it would grant a different power to the boss.
That's the basic premise. If you want a fight to be long, keep it interesting with different mechanics. This can be something like I've described above, or a "phased" fight (i.e., the boss has several different stages or modes), or something like a very specific effect he/she/it is vulnerable to for a killing blow. It forces you to get creative, but there's no way around that; if you just rely on what's in the source books, well... the players have read those just like you have.
Best Answer
A normal werewolf is, as you yourself have noted, definitely out of the question for a usual party four of L1 adventurers. With its multiattack, it has a good chance of knocking out even the beefier members of the party each round. Add to that the werewolf's damage immunity to non-magical attacks from non-silvered weapons of normal damage types (Bludgeoning, Piercing and Slashing). You can end up with a situation where your PCs can't do anything because they get knocked out too fast, and even when they don't, only casters can hurt the beast.
Simple Solution: Tone down the werewolf
The easiest way to adjust monster levels in DnD 5e is to use the existing stats of a lower-level monster. In this case, I'd recommend using the stat block of the thematically similar 1/2 CR monster Jackalwere (DnD Beyond) paired with two wolves as minions (DnD Beyond). Take out the Jackalwere's Sleep Gaze ability, because it's definitely a Jackalwere thing, not a Werewolf thing. The Pack Tactics ability, while not a part of the Werewolf's stat block, can stay, because it makes good enough sense for a werewolf preying with their wolf buddies.
The inclusion of two wolves as "sidekicks" helps to make the encounter feel more like a boss encounter (1/2 CR isn't enough for that on its own) and also adds a desirable element of progress to the combat: taking down enemies is more satisfying than simply whittling down one bigger foe's HP pool. The Jackalwere shares the Werewolf's immunity to non-silvered, non-magical attacks that deal one of the three usual damage types, so the inclusion of minions gives your non-magical characters a chance to be helpful despite the main boss being immune to their attacks.
However, you should give your party a silvered weapon. I wouldn't advise arming them all with silver blades, because the moment everyone has a silver weapon, they stop feeling special and cool.
If you plan to use Werewolves later in the campaign, it might be helpful to describe the reskinned Jackalwere as scrawny, sickly or otherwise weakened. This way, when you get to the actual CR 3 werewolves, the players know not to expect them to be as weak as the Jackalwere.
The resulting encounter with 2 Wolves, 1 Jackalwere still counts as Deadly for a party of L1 characters --- appropriate for a boss encounter. Try not to drain their resources too heavily before the encounter, if you don't want to have character deaths here.
Bold (and Dangerous) Solution: Arm the players up
If you really want to use the actual CR 3 statblock, this is another way you can try, but be warned: this is a solution I really don't recommend doing at L1 for beginner GMs. That said, I think it's still worth documenting for future use:
Arm your players up with magic weapons, wands or other tools that make taking out the Werewolf possible for a party of L1 characters.
I have done this myself: the party was facing a monster above their usual paygrade, and I gave them a wand that could kill the monster. It worked, kinda. The balancing is fickle: the encounter was resolved quickly by unloading the wand into the monster, and you'll want to avoid that because it feels like wand did most of the work, but you also want to avoid the item(s) given being too underwhelming to actually help your party beat the beast.
You also want to give every character something meaningful to do during the combat, which is tricky if success is too heavily rooted in the use of a few magical superweapons. Finally, there's the issue of "rocket tag": if the party has a magical item that can easily slay the beast in two rounds, but the beast is powerful enough to slay the party in two rounds too, the encounter will be largely decided by the initiative roll.
The balancing act is so hard that I would recommend you to go with the Simple Solution for now. Experiment with the magic items later and cautiously, once you get a hang of the level of challenge your players can handle.
Different Solution: Level Ups
Design the adventure so that the party is no longer first level when they encounter the Werewolf. A well-rested L2 party has a much better chance against one than a L1 party does, and for an L3 party you probably want to pair the Werewolf with minions just to make sure the combat is still challenging enough to feel like a boss battle.
The easiest way to do this is to grant the party levels at appropriate points during the story leading up to the Werewolf encounter, eg. minibosses or other tense encounters. You can also use the traditional XP approach and give the players enough encounters and other XP-awarding challenges to level them to L2 or L3.
Just remember that the damage immunities of Werewolves are still very punishing to non-magical melee classes, so hand out a silver weapon when using this strategy, too!