You first need to be maximum level (level 80 currently). You also need some basic raiding gear from running heroic and regular instances. Most of your gear should be Superior (blue) and Epic (purple). Good (green) gear pieces should be few and far between. One easy way to get decent starter epics is to PvP in Wintergrasp or in battlegrounds.
You should also ensure your rotation is the best it can be. The WoW class forums and Elitistjerks.com are both excellent places to read up on the best rotations that yield the highest DPS (Damage Per Second). Guilds will evaluate you by your ability to DPS, heal, or tank. Low DPS means you may not get accepted to the guild or invited to raids where DPS is critical.
Unless you already have a guild in mind look on websites such as warcrafter.net. Here you can look at every guild on your server, their ranking, and what raids they are doing. You don’t necessarily need need to be in the highest ranked guild. Typically there are a least a half dozen or more top guilds that are working on the newest content. Guilds farther down the list also do raids, but they may not be currently raiding the latest and most difficult content. They are worth checking out as well, since older content is fun too. I started off in a smaller raiding guild and had a blast with them.
Another way to find a guild is to talk to the people you do 5 mans with. If they seem like people you would enjoy raiding with ask them about their guild, and if they raid.
Finally most guilds will have a website and an application process. If not, just do a /who [guildname]and ask a random member who you can talk to about joining.
Here are some important questions to ask to ensure the guild will fit your needs:
- Raiding times – You want to ensure you can be on when they are raiding. Different guilds raid on different days of the week and different hours of the day.
How often do they raid – Is it the same time every week? Do they raid every week or just every now and then?
How is loot distributed – There are many different loot methods and some are fairer than others.
Remember, you play WoW to have fun. Make sure you enjoy raiding with your new guild, if not find a new one.