Skateboard decks width is probably one of the most important factors to consider when buying a deck. Decks typically range from 7.25-8.5″. 7.5″,7.75″, and 8.0″ tend to be the standard widths if most decks and the general rule is that the bigger your feet are the wider the deck you should get. Thinner decks are slightly easier to flip, however they are harder to land on due to the decreased surface area. This is a tradeoff you have to consider, but the typical rule of thumb for younger guys is that you get wider decks the more advanced and larger you get. Street skaters typically also like smaller decks as they are easier to navigate and carry around in crowded cities and areas.
Keep in mind how tall you are when choosing a board width. Also consider the kind of style you wish to skate. Most technical skaters (the ones who incorporate advanced flip tricks... like Rodney Mullen) tend to favor boards between 7.5 and 7.75 regardless of how tall or small they happen to be. The upper hand of having a thin board allows for quick rotation. The other side of the spectrum is the "go big" skaters (Jamie Thomas is a classic example of a "go big" guy). Most "go big" guys favor size 8.0 and up. The advantage of a wide heavy board feels more stable under your feet while in mid air and while landing (especially those who have big feet). If your a kid you might want to just stick with a 7.66 or smaller.
Remember don't get those cheap skateboards from Walmart. They won't hold up to any skating and can break from a single ollie that is landed improperly. I know they are cheap, but 2 cheap boards a week for a few weeks adds up to the price of 2 or 3 expensive boards. These boards are also heavier and slow which makes them much more difficult to do tricks on.
Decks come in a variety of shapes and sizes. You can choose a mini-board, a cruiser, a drop-through, or a standard deck for your favorite set of wheels and hardware. Widths range too from a few inches with a 22-inch board to the wide size of a true cruiser. Some decks are flat as possible while others are significantly concave for optimized turning. You can buy a deck with artwork and grip tape already attached for a quick install or you can get a bare-bones wooden deck to truly customize.