Firstly, you must look at what you want out of the board. Are you a beginner? Are you wanting to do jumps and freestyle? Or are you wanting an all-round board for the whole family to use.
With these things in mind there are generally two types of construction in kneeboards.
Plastic boards are good for beginners as they are very buoyant, cheaper and always have a hook where you can place the handle to help with strapping in and getting out of the water. Plastic boards tend to be more durable and easier to ride. If your looking at a beginner board that will be easy to ride and get you into the sport plastic is the style for you.
Fibreglass boards are performance based and thinner. They have a tinner profile and more aggressive shape (see shape section). They have more grooves on the bottom and sharper edges to hold an edge into the wake a lot harder and because they are more solid they pop of the wake a lot more aggressively than a plastic board. If your intermediate or above and wanting to start doing tricks and jumps then this is the style of board for you.
Shape is another key factor in choosing a board.
Tip and Tail
A broader tip and tail make the board more buoyant, and gives you more stability and drive. However, you lose turning ability. These styles of boards are perfect for beginners.
Elliptical and sharp pointed boards are a more aggressive style you generally see this in fibreglass boards. This shape enables you to hold a sharper edge and cut harder so you can pop of the wake at a higher speed. These boards are perfect for tricks and tend to suit an intermediate and above rider.