It's frustrating going into a raid where the tanks can't hold aggro, the healers don't know how to heal, the DPS stand in fire or just don't bring down the boss fast enough to beat an enrage timer. Usually there's one common factor though – lack of experience. Here's a few simple ways to overcome this obstacle if you find yourself in one of those situations.
Learn as you level. It's incredibly easy to solo your way up to level 80. You could get all the way there without grouping up with anyone, in theory. Many people actually do this. The problem with doing this is that level 80 dungeons are a lot tricker than their lower level counterparts. The skills you hone from doing 5-man instances while leveling will prove invaluable when you hit 80 and join your first raid or heroic. If you're a tank, there's a lot more room for error if you're in the Deadmines than in the Halls of Lightning. If you lose aggro on a few mobs in the Deadmines, it may not cause a wipe. But in the Halls, those mobs are going to hit a lot harder and it's going to be a lot more frustrating for the entire group. Practicing holding threat as you level through 5-mans will pay off when you're tanking heroics at 80. The same goes for healing, or watching your threat as DPS. If you're still leveling in Azeroth, you probably don't need to worry about being specced as a tank or healer to fulfill that role in a dungeon. At level 40, you have the option of dual speccing, which may help out substantially as you reach the higher levels. 1,000 gold isn't nearly as hard to come by as it used to be, so gold should not an issue.
Use a DPS meter. Recount is one of the most widely used DPS meters, and I can't recommend it enough. If you're a DPS class, it's important to be able to gauge your DPS. While DPS is not the only important factor in a raid, it's still necessary to monitor. If you're only doing 1,000 DPS at level 80, that's a big cause for concern. It's far less embarassing to find out on your own that you need to pick up your DPS, instead of heading into a raid and finding out the hard way from the rest of the raid! Even if you're not in a DPS role, it's a good idea to make sure the rest of the group is performing well. If you are wiping to an enrage timer because of a couple of low DPS, it will help to find out who needs some coaching, and get them some assistance.
Seek out a mentor. If you're new to healing, find a dedicated healer from your guild and learn from them. Find out which spells you should be using in which situations if you're not sure. If you can't find a good mentor in your guild, you can always look outside your guild. Perhaps there was a really good healer in your last PUG that might be able to help answer a few of your questions? The same goes for any class/role.
Stay informed. Whatever your class or role, there are many blogs that can keep you up to speed with all kinds of useful information. Taking the time to read what others have to say about your role, or a specific boss encounter, will better prepare you for your raid. Taking a few minutes a day to read blogs is a great idea for raiders of all skill levels.
Watch the boss fights. You can look up any boss fight on YouTube and watch the video. This is incredibly simple to do and does not need much of an explanation!