There are six main guilds in the Disc, but I've included one extra one as a sort of footnote. The information on the Thieves Guild is the most comprehensive, since I know a LOT about being a thief...:) The other Guild I know a bit more about is the Priest's Guild. Under each section I've included links to some of the best specialist sites I've seen about the respective Guild, so between all of them you should be able to find out everything you need to know about joining the guild you want. Have fun! :)

NOTE: I recommend you visit the Basic Advancing page before going through the guilds - there are some basic concepts such as TMs and bonuses that you should know about first. If you already know about this, or you've visited the page, ignore me.