What You Need
To start with, you had better have a good computer. Lag, poor resolution, and heat prone laptops are all bad for WoW. You need the top performance around if you want to do well in this game, especially in raiding and PvP. Next up, you'd better spend some time developing the tools you'll need to get through the game.
That means a whole lot of add-ons. If you're an auction house user, get yourself Auctioneer and Auctionator. Add Postal for easy mailbox access. Boost your class with the specific class add-ons you need for your character, and get the tools you'll need for your raiding experience and PvP in the form of TitanPanel and CT_Mod. These are all great ways to boost your overall experience when you use the game's many different interface options.
Finally, there are people who want to know if they need a good guide to get to Level 80 and beyond. Personally, I say it's up to you and what you specifically want for your character. There are plenty of good gold guides, levelling guides, and PvP strategies out there to help you level up. However, if you're even remotely serious about getting to the top, you'll want one of the high end all-in-one guides like Warcraft Blueprint – complete with levelling guides, gold, PvP, and class information.