Read up on the game on the various forums from all the different sites.
Check out the main site: www.worldofwarcraft.com
Read up on this site, on Warcry's site, on IGN's site on all the others.
Come up with questions and ask them (prefarably ones not asked before
).
Then once you've done your research, decide then.
Of course people here will say to buy the game. It's up to you to decide if it's the game for you.