A fast lens is a good idea - and a Nikon
- but they can get fairly spendy. This won't likely be that big of deal in the soccer field, but gymnastics and other indoor activities usually take place in an environment where light is at a premium. Your eyes may tell you that it's pretty bright, but really, it's not all that much so.
Another thing with fast moving sports like auto racing a monopod is a good idea. This is even more important as you try out some slow shutter speeds and panning effects. And the photo course is not a bad idea, either.