What lens to choose for my Nikon? Help me please :D?

I have just bought a Nikon D300s, and I need to find the best, affordable lens for it. I mostly take pictures of indoor and outdoor sports (football, baseball, etc) and ceremonies and so on. I'd like a lens with constant aperture though. (But that doesn't matter that much) Since I'm new to this it'll be nice if anyone could help. :D