First you have to find the right weight, this can be done by throwing a few different pound balls and find what you are comfortable with, the next thing you want to do is find a ball that fits your throw such as straight, mild curve, or heavy curve.
Select the correct weight for your style. Heavier (up to 16 pounds) is better, but only if you can swing it comfortably. Have it custom drilled for your finger size and for the amount of hook you want. The ball contains a cone shaped internal weight. Where the holes are drilled determines the amount of hook.
The first thing to decide will be the weight of the ball. Pick a ball the feels comfortable for you. Secondly, some balls have different coatings that will spin (called Reactive Resin), or will be plastic which will not spin, and are best for beginners.