A prime lens just means that it is a fixed focal length (i.e. there is no zoom to it). They are usually better in low/tricky light situations. They are usually better b/c they do not have to make any compromises designing the lens when the zooming is removed. They normally have much wider apertures than zoom lenses that allows a quicker shutter. You do narrow your DOF with those wide open apertures though, so you have to be careful to not go too wide open for the situation.
You would have to know what focal length you need before you buy the lens. That all depends on how physically close you can get to the action. The most popular one seems to be the 50mm, but that might not be enough focal length for the sports. Somewhere in the 100s might be better. Only you can determine that. They tend to get more expensive the longer the focal length. You can find manual focus 50mm ones for $30 used, but manual focus would be tough for sports. I believe there is still a rebate on the new 50mm, but they have been hard to find.
Kevin