Take the door off its hinges, lay flat. Try to discover what the cause of the problem is. Is the door uneven? Has part of the veneer chipped off and is scraping the floor? Is there a metal kickplate that has become partially dislodged and is lower than the door edge? If there are pieces of veneer chipping off, simply sand those away, or repair a metal kickplate that has slipped down below the door edge.
Step2
For a very uneven door, use a plane to shave off the part of the bottom of the door that is sticking. Sand and repaint if necessary.
Step3
For a door that just sticks a tiny amount, consider simply sanding the area down with an electric sander. Repaint afterwards if the paint chips off.