Photography for sports?

I really want to get into sports photography (geared more towards martial arts and weight lifting) and i'm wondering what lense i should get first.
ill be shooting with a nikon d3000 and the majority of my shots will be indoors and shot in smaller places

I use a 24-70 mm f/2.8 lens when shooting wrestling, martial arts, etc.

Indoors, you are going to have to use ISO settings of from 3200 to 6400 in order to use shutter speeds high enough to stop the action without blur and at those ISO settings on your D3000, you are going to encounter some noise in your shots.

When you can budget for a camera which performs better at high ISO settings, you will find less noise in the images at those high ISO settings.

I use a Nikon D800E when shooting indoor sports and it performs nicely at 6400 ISO. You will have to test your camera at high ISO settings. Shoot in RAW and use the noise reduction tool in Lightroom to get rid of much of the noise in your images