Lysol Power Scum cleaner in the blue trigger spray bottle with a Magic Eraser or a scrub brush.
For more serious issues Lysol Lime & Rust Remover Toilet Bowl cleaner, comes in a black squirt bottle. Scrub with Scrub Daddy or Mr. Clean Magic Eraser.
Wet & forget shower for a slower, no scrub solution. You can use 2x a week to accelerate the cleaning process then once a week to keep it clean, as long as it isn't marble.
Here's a way to do this without killing your back.
Go to Dollar tree and buy a blue bristled broom. Get a 32 oz empty water bottle and add 6 oz blue dawn dish soap, fill with white vinegar. Next make holes or sits in the cap. After a quick mix, squirt atound your tub/shower and let it set. Then take your newly dedicated broom to this task only, start to clean your stall.
Rinse. You are done repeat as often as needed.
USE A PASTE OF BAKING SODA AND VINEGAR ALONG WITH A BIT OF WARM WATER AND USE A SPONGE TO APPLY AND RUB A LITTLE. RINSE WITHN WARM WATER TO REMOVE ANY RESIDUE.
This post has great tips for cleaning a bathtub.