Hmm. In my haste I realized that I didn't answer your question very well.
Basically around lvl 8 or 9 your quest journal updates. You will see additional details in your Hallmark quest entry (in quest journal). Read this carefully and you will know where to go to become a crusader.
I'm assuming the process is similiar when you are close to lvl 20. Good luck.
Edited, Thu Nov 11 00:17:12 2004 by Tye