You can paint with the Epoxy kits made for that sold at Home Depot. It's a temporary fix but will buy you time to save up for replacement.
The bathtub itself, yes?
I've used a paint to refinish one in the past. Here's what I used and it worked well: https://amzn.to/2py8tRD
A bathtub refinishing kit costs about $15 and comes with detailed instructions. But the job really depends on the cleanliness of the tub. If nail polish, polish remover, hair spray, hair color, or any other caustic material has ever come into contact with the tub, there may be residue or damage, which needs to be removed or repaired before a redo. Concrete, cement, grout, spackle, paint, adhesives and more can harm the surface of the tub. Just clean as normal, first, then fix any problems. Then use the kit, exactly as instructed. Take photos as you go, and post on Hometalk! Best wishes, Jewell.
This might help: https://thehoneycombhome.com/transform-your-bathroom-with-sink-paint/