First, open the windows and use a fan for ventilation. Clean off any mold and mildew with a bleach solution, rinse that off, and follow it up with Lime-A-Way to get rid of any water damage. (This is a good idea even if you're not planning on painting, because it'll make your tiles look good as new!) Scour the surface with sandpaper — the gloss can make it tough for paint to stick so this step is important. Mix the refinishing kit and apply the mixture with a roller. Use a small foam brush for the corners, then let it dry for an hour before applying a second coat. You'll need to leave the surface to set for three days, then re-caulk.
Get the Rust-Oleum Tub and Tile Refinishing Kit at Home Depot for $25.97.
Get the full tutorial here.