In order to shoot low light action, it helps if you understand the basics of exposure, and how the three settings (aperture, ISO and shutter speed) interact. You can Google that for several fine primers online.
Then do this when you arrive at the venue:
1) Set M mode
2) Select a shutter speed that will stop action. For fast sports like HS basketball 1/800 is preferred and 1/500 is kind of the floor before motion will creep into shots. Some sports might be OK with slower shutter speeds if there are periods when the athletes are paused.
3) Set the aperture wide open, f/2.8 in your case.
4) Pick a starting ISO (I'd try 3200 to 12800) and take a test shot. Check the image and histogram. You want to push white tones in the scene all the way to the right of the histogram, on the edge of blowing out but not quite blown, so keep changing the ISO until you find that through test shots. Once you find that ISO, you are all set.
If this places your ISO higher than you like for the camera you have, there are options:
1) Apply noise reduction with software. The NR in DPP (free!) is pretty good.
2) Get a faster lens.
3) Get a body that makes less noise at higher ISO.
4) Set up strobes and remotes and light the whole space. This requires a different approach to all settings (not the same as detailed above) because now you have to slow down the shutter to sync speed and then stop down to eliminate ambient light.
The latter three choices can get real expensive real fast, and in a lot of cases it isn't possible to set up lights or lighting is banned for the sport in question (like gymnastics).
