Turn you shower on to hot. Close the shower doors or curtain. Allow the hot water to run for 10 minutes or so. The wet heat will help to soften grime, soap scum and dirt.
In a bucket mix 1 cup of white vinegar, 1 cup of ammonia and ½ of a cup of baking soda. Fill the rest of the bucket with hot water. Do not use harsh chemicals because it will remove the finish. Do not use abrasives because they will scratch the surface. Be sure to work with ammonia in a well ventilated area.
Use a sponge or old towel to scrub the walls.
Rinse well with plain water. Dry the walls with a clean soft towel or rag and apply a coat of fiberglass wax. Use care not to get the wax on the floor of the bathtub or shower because it will make for a dangerous, slippery surface. The fiberglass wax will make the dirt, soap scum, grime and hard water stains less likely to stick. Waxing should be repeated every 4 to 6 months depending on use. Fiberglass wax is available at hardware stores, home improvement stores, boating and RV stores.