What's your style?
There are different styles of skateboarding. You can pick one type or do them all. The style you pick when you first start out will determine the type of board you go out and buy. Each type requires a different board. I've learned throughout my life that most boards are the same. Brand names rarely mean anything, it's all about the SHAPE of the board that matters. No matter what style you pick, you're going to want the right board for the job.
Transition/Vert skateboarding
This is the style that started everything. If it weren't for this style, there would be no skateboarding. All you need are some good transitions, like an empty pool in the picture on the right (photobucket) or a vert ramp like on the left.(espn.com) A wider board is needed for vert/transition skating. Somewhere between 8 and 9 inches wide.

Street/Technical style
This is today's most approachable style. Everyone can do this. all you need is a few obsticles and some guts. A narrow board is prefered. Somewhere between 7.5 inches and 8 inches is best suited for street skating. Eric Koston (below)(transwolrd skateboard magazine) is arguably the best street skater in the world.
Longboarding
This also a very approachable style. Go to any college campus in the world and you can find someone cruising along on a longboard. They are the bicycles of the skatebarding world. They are used for transportation and cn travel at speeds up to 45 mph. You can go to most skateshops and pick one up. It's an entirely different set up for this board. the look is different, but the parts are the same.(photobucket)
If you've decided what type of skating you want to do, then the next step is to build a board.