Denise, this is an easy fix if the door is painted, tougher if it is stained wood. Basically, you fix it just like a hole in drywall. If the hole is small, mix up some Bondo for painted wood or Minwax Wood Filler (2 part) for stained and put it in the hole with a putty knife. Overfill it and sand it down when the Bondo or Minwax has set. Then prime and paint or stain and finish (varnish or polyurethane) to match the rest of the door. It is tough to color match stains on the Minwax and the best simple suggestion is to try a darker version from the same general color range. Color matching stains is a large and very involved subject and a lot of trial and error might be in order before you apply the clear finish.
If you have a large hole, cut a piece of 1/4" plywood just big enough to fit the largest dimension of the hole. Drill 2 holes near the center of the plywood. Put string in one hole and back out the other, then tie the string together. Insert the plywood in the hole and use the string to hold it in place while you apply the Bondo/Minwax. Then continue as above.