http://www.tankspot.com/forums/f92/3...any-guild.html is a must-read for you I think.
If you are truly serious about it, it might not be as hard as you might think to get accepted, good and competent DPS is very much needed nowadays.
I personally recommend hunter, they are an allaround good class with very good potential if you take the time to read up on and gear them (/hint Warcraft Hunters Union). Then again this really goes for any class.
My suggestion is to first define to yourself exactly what kind of guild you are looking for, do you want to raid 10mans/25mans/both, what times can you commit yourself to raiding that would fit the guild's schedule, etc etc. Then hit up your server's realm forums and start looking around for recruit threads, check out guild sites/progress/armory. Talk to the officers ingame, you have nothing to lose and often a quick chat will tell you more than any "Guild Application Form".
Most importantly, stick to the class you decide to make your main, make sure it will be the class you love to play and be dedicated to it, no guild takes it happily if people change their minds about their class every 2 months.