crusader questFollow

Well, the instructions for who to talk to is in the manual that came with the game.

You talk to an NPC in South Qeynos. He's a dwarf, and you can find him right next to the fountain. I can't recall his name off the top of my head.

He'll have you run a handful of quests, at which point you'll be able to become a Crusader.

I have to say though, you could have found this out by just doing a short bit of reading or talking to NPCs.