Tank Harder, Heal Better, DPS More.
This is a little guide I put together and posted on a forum somewhere. It's a little summary of things I think everyone should do. If you've done everything on this list, you're likely in the top half/top quarter of your guild. It surprises me how little effort it is and how few people make that effort.
First of all. If you haven't already while you were levelling your character, level Cooking and Fishing. Also level First Aid if you have no healing abilities. Food gives great buffs at maximum level (40 stamina + a stat you want). It's essentially free. Fishing makes it easier to level Cooking as most fish you catch can be cooked, whereas meat has to be farmed with (some) risk to yourself.
Look up stuff about your class. Look up stuff about your spec. For every possible spec there are guides out there that tend to cover Talents, Glyphs, abilities (and their use), stat priority, enchants and gems to choose etc. You may learn something, it may tell you to try something you never even thought of. It may just show what that ability is for you thought was useless. Even if you learn nothing, it's a good thing to hear someone elses view on what you want to be doing. Guides also tend to suggest what Professions might be useful to you.
Get some professions. Most professions nowadays have exclusive buffs. Leatherworkers get wrist enchants that are much better than what's normally available. Enchanters get ring enchants. You may even be able to make yourself some starter-epics, that will help get you raid ready.
Gear up. A guide usually tells you what the desirable pieces are and where to get them. If in doubt, look at www.wow-loot.com, it has very comprehensive lists of what equipment drops where and who it's useful for. Each class/spec (far as I've seen) has 4-5 epics to get from Northrend Heroics, some pieces from a profession, the rest of the epics come from raiding. It might be different on your server, but on mine, LFG works. Use it.
Read up on tactics. theres are some excellent sites out there with info on how to tackle most fights. www.wowhead.com, www.wowwiki.com and many more. Knowledge = Power.
Lastly, be prepared. Carry food/drinks, ammo, bandages, potions, elixirs, switch-out gear and reagents if any of your spells need any. Repair your gear before you go anywhere.
I feel a little better already! Have fun.