Preparation is very important. Getting a good surface to paint on is vital.
Old kitchen and bathroom tles can be grubby with grease or limescale. It's important to remove this with limescale remover and a thorough clean with detergent.
Using a very fine sanpaper give the tile a good rub down then rinse and dry the surfaces.
The tile are now ready to prime. Follow the instructions on the tin. I usually apply one to two coats of primer. When fully dry, one to two coats of top coat (I usually use a satin finish in the colour of my choice)
On a smooth tiles surface you want to aim for a smooth paint finish. Using several thin coats of paint rather than putting it on thick will give much better results. Give a light sand down and clean after each coat.
I like to apply the coatings using a sponge roller AND a brush. A 4 inch sponge roller costs very little and is a great for appying a thin even coat. As soon as you have applied the paint with the roller go over it very lightly with a fairly dry 2 inch brush to flatten down the roller marks. (other wise using only the roller you will get a slight "orange peel " surface.