Hold your line, the faster riders can find their own way around you. Be predictable.
Don't cut the track, and do not cross the track to pull off. Plan ahead, and move over gradually before pulling off. Often the best place to get off of the track is coming out of the corners.
Don't try to keep up with anybody, but if you can then great.
Don't jump anything that scares you, yet.
"I'm tired, but I think I can get one more lap" is a bad idea. If you get tired, take a short break.
Most important thing, relax and have fun.