1/4 to 1/2 cup baking soda
essential oil (optional)
Put baking soda in a bowl (cereal size is good). Add 10 drops of essential oil if using. I use sweet orange oil which smells heavenly while scrubbing the tub. Add a tiny bit of water at a time mixing with a spoon until a paste forms. Smooth the paste over your bath tub and let sit for 5 minutes. Use sparingly as you aren't going for thinkness on this. Come back and scrub your tub clean.
*this requires the usual amount of elbow grease you would exert while scrubbing the tub with any other product. it is not a "magical" toxic chemical that will strip the dirt away before your eyes. just a cheap and environmentally friendly way to clean your tub.