For everyday cleaning of fiberglass shower stalls, use white vinegar, on its own or mixed with shampoo or dish soap. For a more thorough cleaning, scrub with baking soda or Borax.
If stains do not come off after scrubbing, cover them with baking soda and a paper towel soaked in vinegar, let sit for half an hour, and then scrub off with a nylon brush.
Rust and limescale require commercial acidic cleaners or acetone. Be careful not to use this kind of cleaners too often, due to their acidic nature, and make sure there is enough airflow in the bathroom while cleaning. Cleaning gloves are necessary in dealing with such products.
For old and stubborn stains, try using no-fume oven cleaner. Coat the stall with oven cleaner foam, from the floor up, and leave it for at least an hour after scrubbing off the remaining stains.