Wash the door and frame with a solution of 1/4 cup trisodium phosphate and 1 gallon warm water. Trisodium phosphate is an alkaline cleaning product used by professional painters to quickly cut through grease and grime. What appears to be damage to the door may actually just be dirt and wear. Once the surface is clean you can more accurately assess the damage.
Hammer small carpenter's nails into the wood frame every 12 to 18 inches if necessary, if the frame is sagging or loose. Apply wood glue to any loose joints and tighten the screws in the hardware that holds the door.
Apply wood putty or epoxy to cracks or gouges in the wood frame with a putty knife. Smooth the putty so it is level with the surrounding wood and allow it to dry. Sand it with a fine piece of sandpaper.
Prime the door and wood frame with a high-quality wood primer and paint it. A fresh coat of paint can hide many minor flaws in a door or frame.