I just started by joining the dawn guard and they gave me a crossbow. Which is better bow or crossbow? Where do I get bolts.. or the armor that they wear?
Also I was just turned werewolf.. I thought Dawnguard had new perk trees for wolf and vamp but I don't see one..

Crossbows are more powerful than bows, but they take slightly longer to reload and they're louder (important for a sneaky character). I'd say go for either a crossbow or a dragonbone bow - they're comparable.
You can buy bolts from Sorine or craft them in Gunmar's forge.
Armor is scattered around Fort Dawnguard, you'll also get a set from Gunmar after you recruit him.

The new perk tree is accesible only when you're in beast form.

crossbow by a long shot

I personally prefer crossbow. A lot more damage and with high sneak no one notices any noise anyways lol

personally I prefer crossbow for close-quarter combat and dragonbone bow for open range.

Legendary Dragon bone bow with Daedric arrows, with all the archery perks filled out.

Well a short to mid-range shot !

My character made bolts at Gunmar's forge and later was also able to make them at the forge in Windheim. Haven't tried the others, yet.

!.) My legendary crossbow actually does the same amount as my Daedric bow. Crossbows are cool and they look awesome on your back, but I've been a huge fan of regular bows since I first played TES so many years ago. So it would be hard for me to choose a crossbow over my beloved bow! I use both of them anyway.

2.) Crossbow bolts can be crafted at smithing forges just like you can make arrows now with Dawnguard. You can also find stronger Dwarven bolts in Dwemer ruins. I have over 200 Dwarven bolts now just from searching.

3.) Talk to the armorer and he can set you up with your set of Dawnguard armor.

4.) As for perk trees for Werewolves and Vampire Lords, you have to turn into one then hit the inventory button and it will bring up the perk tree. You don't use the usual perks earned from raising levels. You earn perks by killing enemies, feeding on them, etc.

Hope that helps!

Crossbows take a little longer to reload, but the best part about them (in my opnion) isn't the power, it's the fact that when you have it on your back all you have to do is pull it out and shoot. No knocking an arrow, pulling the bowsting lining up and firing. You pull it out and pull the trigger, click thunk dead. After the first shot though it's more effective to pull out Zephyr (the dwarven bow that shoots 30% faster) with the quickdraw perk and you draw the bow over 50% faster, making your DPS skyrocket. To sum up what I've said I use a crossbow to initiate combat then Zephyr to perforate my enemies.