How can I paint my sink top in bathroom?

You need to use porcelain paint, its not too difficult- http://homeguides.sfgate.com/repaint-porcelain-sinks-21295.html good luck!

Remove the faucet, clean the surface using a really good cleaner, scour with a steel wool. Repeat the clean and scour a couple of times to make sure your sink is properly prepped for painting. Tape off the areas you don't want painted (aka wall and drain) and then apply your paint carefully and thinly with a nylon brush. Wait until completely dry between coats and make sure it has time to cure when done before removing tape. You got this!

Just use the specialty paint available for this type of job. I have done it and love it. Just Make Sure you clean, clean, clean before you paint