Hi Arandora,
The tiles are best cleaned with a kitchen scourer and some sugar soap, they then need to be left until they are completely dry.
As tiles are so shiny they need keying to enable the paint to stick, this can be acheived by sanding lightly with 200 grade sandpaper but it causes a lot of tile dust, which you do not want!
Tile primers are probably a better alternative and easier to use,Here is some tile primer -
tile primer
The tile primer should be applied and then left for as long as possible, preferably 24 hours, then a light sanding another good clean of the tiles before painting!
A small gloss roller is ideal for painting tiles as you do not get any brush marks with a roller, be sure to roll in all directions, not just up and down. Two thin coars are better than one thick coat!
Follow the manufacturers instructions for the tile paint, some require 6 hours between coats!
A friend I know painted her tiles with two coats of normal white undercoat and a top coat of liquid gloss, they look fine and have been ok for the last 2 years, whether they will last or begin to peel, I do not know?
Good luck