Ceramic bathroom shower tiles: Can it be painted over?
My experience with tile painting is that it's cheaper, easier and better looking to re-tile.
If you are determined to try it; you can buy silicon carbide (wet or dry) paper to fit orbital sanders, I would buy about a 120#. Once the surface is scuffed, go with a high quality, 2 part epoxy paint. I like the marine stuff, a gal. should go for about $70-80 plus hardener, $35-40, plus reducer, $20-25. It's not cheap paint.
Let me know if you decide to try it, there are lots of tricks and warnings.