go to your local DIY and buy ceramic tile paint. Its usually with the specialist paints like radiator paint, magnetic paint etc. I have never done it but understand that the secret of success is meticulous preparation which is why I've never done it
I've painted tiles with great success much to my own suprise. I used a special tile undercoat & then just a good quality gloss topcoat but I believe you can now get a paint to do it all in one. I used a small fine sponge roller which gave a nice finished effect, but as woofgang said you must make sure tiles are completely grease free. Be warned however, that even when the paint feels dry,for weeks afterwards if you put anything heavy on painted tiles it will stick!