what philly said + you should also play around with different shutter/appeture settings as well as different focus points to get rid of the stationary-ism (is that a word?) this is just my opinion, so, take it as you will, but. stationary pictures are flat and boring unless its in photo travel.
First off, Welcome to GTP. Enjoy your stay! You've got a fairly nice start to your gallery. I agree with philly and es, you need to first brighten your photos up, they are too dark to see any real detail. As es said, you should also play around with the shutter speed. In your racing shots the wheels should be blurred being as the car is moving. Not bad though, try adding those little things in and you should have a much better outcome.