Now that I've had my fun, let's get serious - how would I play a rogue at 90? Or a mage, or a priest, or another class that I'd never played before? Well, there are resources out there -- WoW Insider has a rookie guide, for starters -- and there are other sites like Wowhead and Icy-Veins that can and do give you an overview of what your new class does.
There's also a very useful resource built into the game itself. Your spellbook has a great deal of information for you about what your class can do and how to go about playing it, that can get you pointed in the right direction to begin play. We're assuming you're a returning player who has not played in a while or a new player just getting started with this article - established players starting a new alt may already know much of this.