Its actually very simple. I will give you a shorthand guide of what I did last time I was serious about joining a raid guild.
Step1
Make sure the character or even characters you are signing up with, are 100%. Ie. make sure if you have the latest and best pre-raid gear, especially if you can get it from rep. Then, make sure you have the best gems and enchants on all the gear, although you can skip the epic gems for now, but if you have lots of gold, go for it.
1. Best pre-raiding gear -> The 3 new heroics and LFR DS is a great start
2. At least blue quality gems, if you have the gold go for epic quality gems
3. Best enchants over the board
4. Picture perfect reforging, make sure you have your hit/spellhit caps, expertise ect.
Step2
Now here is where it gets interesting, you need to build a list of guilds that qualify for you. Crazy right? But if its not the right guild for you, no point in joining, you will just cause problems for yourself and them.
Things to keep in mind:
1. Raiding times, which days and times are acceptable for you to raid. Me for example, like to raid on Wednesday/Sunday/Monday raiding schedule, and I like to do it between 8pm-11pm South African times. So 3 days a week, 8-11 in the evenings.
2. Progress. Trying to join a guild which is above your progress is a good way to not get a spot, unless you are truly lucky they take anyone or you know someone that will vouch for you above any concern they might have.
So look for a guild which is at your progress level, ie. if you never raided DS but did LFR, its acceptable to look for a guild that is 4/8 to 7/8. Don't go for a 8/8 because they are now going to progress heroics and your gear won't be the required level to perform or even look acceptable and might fail the trial.
So now that you know what you are looking for, head over to
http://www.wowprogress.com/ - do a search on your realm/faction and then see who pops up around your progress level. Once you have a list which falls into your progress level, start reading their websites and recruitment pages, see what their raiding times are, if they are English only or some other language, filter out any guild that doesn't fall into your requirements.
Now that you have a list, you are ready to sign up, but before you do, read carefully what they want you to do, what process they want you to follow ect.
Step 3
So this is the fun part, where you need to sign up, and I think the follow article will be a good read:
http://www.tankspot.com/showthread.php?36277-how-get-accepted-into-any-guild
They explain very nicely how you should post your application form, and its a great guide to give a good first impression. Remember, its a real life person that will be reading your application, and a good impression goes a long way. After you are accepted as a trial, being a good raider, skill and being socially acceptable in that guild is important factors to have your trial turn into a permanent member.
Best of luck.
