The first thing you need to do is to learn how to differentiate the different types of boards. The Drop through boards have a cutout in the deck, this allows the base plates to go straight through and get mounted through the deck. This allows you to stay stable at higher speeds.
The hybrid boards include wheel cutouts, this allows the wheels more room, which makes it a whole lot easier to turn about without suffering any wheelbite while you are doing so. These boards are smaller than the normal longboards, and this often results in show bite, when the riders shoe hits the wheel.
The cruiser is the most common longboard. It looks almost like a normal skateboard, with the only difference being that they have a kick tail. The wheels are a little bigger than normal skateboards, but these boards let you have more maneuverability.
Now based on your preferences, go on and select one of the boards that you see listed. After you have made your choice, go on and stick with it and go out and look for the board that you short listed. It would help to take someone who knows about boards to help you out.
However, you can either head out to a local skateboard shop as well, or shop for your board online. Either way, you will end up with a board that will do just what you want it to do.
Just make sure you don't overpay for the board, since the salesperson might be looking to make a decent profit off of you.