There are many mountainboard models and styles to choose from. The one that is best for you depends on your type of riding style. If you like big turns and lots of speed you should choose a longer board with more stability. If you like launching ramps and freestyle then you would want a shorter and lighter board so you can cut corners quickly and be able to launch the ramp. If you are a beginner,however, you can probaly see if the company has a beginners board, which is often cheaper and made so you can really do carving or freestyle but if you really want to do either you'll probly need a new board.
This is a freestyle board made by mongoose. As you can see it is small and light weight. It also has two tips, like most boards, for switchstance riding.
This is a slalom board also made by mongoose. You can see that it has a longer frame for more stability and carving. This too has two tips for switchstance riding.