Protection... Tanking. Your probably gonna have to ***** yourself out to a guild when you hit 70, just to get the gear your gonna want for the spec you really are going to try to do end game.
Since tanking is what warriors do best, it's a good idea to learn how to tank.
Arms is the pvp tree, and since warriors are kinda weak when it comes to ranged casters and kiting, learning to pvp is a good idea too. Since it isn't as easy for us, you don't just stack tons of resilience and hp, then just nuke nuke nuke.
Fury is the best damage giving tree. It has the best scalability of any tree. This is the tree that has all the better pve dps talents. If you want to be a huge asset to a guilds dps, get good gear first, then spec fury. It also can be very low dps with ****** gear.