Are tradeskills other than First Aid useful?
Yes, all tradeskills are useful. Cooking gives you buffs, smithing allows you to make resistance gear, etc. However, the requirements I listed above are more important. If you have a choice between getting your Onyxia key or getting 300 Blacksmithing, go for the Onyxia key.
How do I find a raiding guild to join?
Join the GuildRecruitment channel, and watch for people advertising in the major cities.
On your WoW realm forums, there is usually a guild progression thread listing the guilds involved in endgame. Most of these threads have links to the guild websites.
As well, keep an eye on the realm forums, and check out the Guild Recruitment forum. Guilds often post to the forums if they are looking for people.
Participate in pickup groups. You may group with a raider, and she may tell you if her guild is looking for people.
What should I look for in a guild?
First, make sure that you can attend their raids. There's not much point in joining a raid guild when you can't make the raids. If the guild doesn't have their raid times listed, ask a member of the guild in game.
Second, make sure you like their loot system and rules. Loot is one of the major reasons for going to the trouble of raiding, so make sure you can live with the system the guild uses. Most guilds do not use the built-in rolling systems, but rather DKP or point systems.
Third, see if the guild is recruiting your class and if you meet their requirements. There's no harm in applying to a guild if you don't, but you are more likely to be successful if you are what the guild needs.
Try and make sure the guild is a good fit, personality-wise. This is often harder than it appears, because you don't really see the true face of the guild until you join. Don't join a guild if you dislike some of the guild members or have had bad experiences with them. You'll be spending a lot of time together, and it can get frustrating at times. Don't be afraid to leave a guild that isn't working out for you. Epics are not worth being unhappy!
How do I apply to a guild?
Most raiding guilds will have an application forum on their website, where you will find a list of requirements and an application template. Fill out the template to the best of your abilities.
Do not lie on your application. It's much better to be truthful, and lying will only create bad blood. If you seem to fall short of their requirements, it's much better to acknowledge it, rather than dance around the question. Also, if a guild asks you about your previous guilds, do not badmouth your old guilds. Be honest, but be classy.
Often, an application will ask you about your gear. It's useful to create a profile at a site like CTProfiles. This way, you can just put a link to your profile on the application.
Finally, only apply to one guild at a time. Most raiding guilds frown on people who apply to multiple guilds. Apply to one guild, and wait a couple days for a response. If they don't respond, contact them in-game and ask politely about their application. If you do decide to apply to another guild, post a response to your first application saying that you are withdrawing it.
I got into a guild, and am going on my first raid! What should I know?
Congratulations! Here are some quick tips to help you on your first raid:
1. Show up on time and prepared. Show up at the instance enterance a few minutes before the raid starts. Make sure you have all the gear you need, and that everything is repaired.
2. Have a stack of heavy runecloth bandages, 5x Major Health potions, and 5x Major Mana potions if you are a mana-user. You should also have enough reageants to last you the entire run. I usually bring twice as much as strictly necessary. It's better to have too many than to run out.
3. Follow instructions from the raid leader. If you don't understand something, speak up and ask a question. Also, it's a good idea to choose an experienced player of your class and follow her lead. Be careful here, because sometimes the experienced player can get away with riskier moves, so try to err on the side of caution. However, watching how the other player positions herself will often help you understand the fights.
4. Try and read up on the fights beforehand. A good source of information is WoWWiki. You never truely understand a fight until you experience it first hand, though. But reading up on it first can help a lot.
5. Don't go Away From Keyboard during the run unless absolutely necessary.
6. You probably won't get any loot from your first run, because you'll have less points than the other raiders. Don't expect loot, and don't bombard the raid with questions about loot. If you have questions or concerns about how loot is handled, ask after the raid.
7. Try to show up to all the raids, especially the learning ones. Don't expect a spot in a very cutting edge raid, because you will generally be outgeared by the other raiders. However, show up and be prepared to jump in if the raid needs you. Let the raid leader decide if you are geared enough. It's very important to show up to both the raids with a lot of wipes AND the raids with a lot of loot.
8. Have fun! Remember it's a game, so don't get so caught up in everything that you forget to have a good time. Blizzard has put in a lot of work into their raid dungeons, and it's worth taking the time to enjoy them.
Okay, I think that's everything. Post if you see anything missing or that should be added.