how to clean grout in bathroom tiles,

hi, it's julie, now onto the next project, i'm about to fit a new suite, well not me, my partner , althought never done one before, so that's going to be fun. never used the hot water tap on the sink for approx 5 years, yes honestly have to fill with a jug from the bath. the tap on the sink stopped working, would not let any water come out, of course, partner, john, said all heating/water would have to be turned off, other words too much hassle /grief, so it got left. anyway i'm about to get new suite soon, yes the old ones ancient, SO YES HOT WATER FroM THE SINK, IT WILL BE HEAVEN. i wanted to place new pvc clading on the walls, again too much hassle, yes he's a mechanic, nothing else. over the years i know how to plaster walls, bricklay, paint, just as i well i enjoy doing jobs like this, i was told just clean the tiles/grout. i will have the job of this, as i dont mind getting hands dirty, do i have to remove all grout ? or is there some special cleaner. any help.

See this.
There are available from most good diy superstore's and depends how bad it is,you can get a grout tools remover to rake out the joints then re-grout with fresh product.
See this.

If the staining is not too bad, thick bleach (the really thick stuff) left on can help quite a lot. Do a test spot in case it does anything it shouldn't, but I've used in this in past on my bathroom tiles and it's brought them up a treat.
Also the DIY shops do a tube which 'paints' over the grouting, it's like a tube of toothpaste. Works well but not terribly long lasting.

I find that a spray bottle of Mildew Remover gets rid of staining on grout. It seems to comprise mainly bleach, and it works a treat for me. Spray it on and leave it for an hour. Then rinse it off.