When i drive, sometimes the ABS light turns on. Then i turn off the car. Next time i start it it isn't on anymore, then it might turn on after driving a couple of minutes. Sometimes it doesn't turn on at all. What could be wrong?

Maybe some air in your fluid or its contaminated. I also had to bleed/flush my old fluid out for new to fix my abs problem... Check for leaks also its not uncommon for your MC or your ABS unit to leak.

It could be a speed sensor. We need to know the antilock codes.