To get rid of the mould you already have, try using Dettox Mould and Mildew Remover. I use this and it is excellent, though I only get a light build up in the top corner of my ceiling above my shower every so often. If you have a bad build up, you will probably need to leave it on for an hour and so and then give the affected areas a good scrub. To help minimise mould in the future, use a paint specifically for bathrooms (don't be tempted to paint over the mould as I once did, it just creeps through!!).