The shower is a place of high moisture. Whatever material you use to cover the walls must be able to withstand not just water, but steam and prolonged dampness. For this reason, tiles or vinyl are usually used on shower walls. Still, you can paint them if you use a water-resistant paint and caulk the edges carefully. Be sure that the walls have been covered with water-resistant green rock and not just ordinary drywall if you don't want to sagging, bubbling or peeling wallboard after a few years.
Mask off around the edges of the shower stall with masking tape and paper. Use masking tape to cover the shower handles and head or any other fixture that you don't want to paint.
Prime the walls first with a high-quality, water-resistant primer. Use a paint brush to cut in the corners and edges of the wall; then roll out the walls with a roller. Let the wall dry for four hours or more.
Spread on the first coat of paint in the same way, cutting the corners with a paint brush and rolling the wall. It is preferable to use an oil-based paint in the bathroom, and especially in the shower. In areas where interior oil-based paint is prohibited, however, use a high-gloss latex enamel.
Let the wall dry, then spread on a second coat. Paint a third one if needed.
Caulk around the edges of the shower stall with silicone caulk when the last coat has completely dried.
Use a roller with a thick nap to enable you to spread a heavy coat of paint. The more paint on the wall, the better. Don't forget to cover the shower floor with a dust sheet. Clean up drips and spots of paint with a cloth dampened with water or thinner before the drips dry.
If you use oil-based enamel, keep the room well-ventilated and, if you are very sensitive, use a respirator.