How do you repair a small dent on side of my car?

If it is a small dimple that has not damaged the paint you can have a person do a "PDR" paintless dent removal.

If it has damaged the paint the procedure is:

1. Clean the surface of wax and grease
2. Sand the painted area of the door, to bare metal at the location of the damage.
3. Fill the damage.
4. Prime and check the repair making sure the surface is PERFECT
5. Paint the door, usually with a base coat clear coat system.

The last time I did my ex-wifes car (don't ask) it took me 8 hours over four evenings.