Disassemble your metal bed, and take it outside. Always work outside or in a very well-ventilated area. You'll be sanding metal and working with spray paint, so you need lots of air flow.
Place the metal bed parts on old sheets or drop cloths to protect the surface underneath from old paint and metal dust and to protect surfaces from the spray paint.
Remove or sand down old paint. If your metal bed has old chipped paint, use a paint scraper and wire brush to remove it. Alternately, use a paint stripper made for metal. Use fine steel wool to prep the metal for new paint, and wipe the parts down with a rag to remove any dust.
Apply primer, which comes in either liquid form or in a spray can. Spray primer is easier and faster to apply. Apply primer to one side of the metal bed and allow it to cure according to the manufacturer's instructions. Flip the parts over, and paint the other side. Prime the entire surface to help the final paint adhere.
Paint the metal bed. Use a spray paint made for metal surfaces, and apply according to the manufacturer's directions. Apply thin, even coats from 8 to 10 inches away. Apply to one side, then the other, just as you did with the primer. Follow with a clear coat, if desired, for extra protection against paint chipping.