Easy, you can either get a mild bleach solution and a toothbrush and plain elbow grease, or buy a bottle of 'grout buster' I think its made by Domestos and it is brilliant stuff.
Hello. I've got some fantastic stuff - it's made by HG, Hagesan and it's called 'Mould Spray'.
I sprayed it on my grouting which was awful, left it on for ages and it's come really clean.
I can't remember where i got it though, sorry.