I'm going to give you a nice, broad subject to work with: lying.
Make a one-panel comic where one of the characters is telling a little white lie, a normal, everyday functional lie, or a great, big whopper. Or it could simply involve lying in some other way.
1. No photoshop.
2. No old comics.
3. No references to the time Space Ghost gave Beck a taco and Beck wouldn't eat it because it was too old.
This is the most exciting thing that has ever happened to me! I'm really looking forward to reading your comics!