How do I paint a bathtub ?

there are tub paints available in a kit form, I don't recall the brand, rustoleum might be one, but there are others.

There are professionals who do this. They should guarantee their work.

Again: Rustoleum Tub and Tile resurfacing kit. They come in White or Almond.

The RustOleum kit gets my vote. 2 person job, follow directions exactly, wear a respirator and vent the area as much as possible.