gas light on,beep when start

gas light come on when I just pump up 3/4 of a tank.
What's going on?
Also right after I start up the car there this loud beep then it goes away in about 2 sec.


vehicle
99 new beetle 2.0l automatic

After the symtom has occur during the one time I'd pump to full tank. I used up all the gas until it almost to the red empty mark then I pump up the gas back up to the 1/2 mark. The warning light and the beep went away. I'm still confuse. Maybe something stuck. But now it's working fine. Let's just wait until I pump up to full tank again. thanks for the sugar info. Maybe I'll try that next.