Hi Can I replace old tiles by putting new ones on top is there any thing special I should do to prepare for this , they are in a bathtub area.:o

I'm no expert on this, but we've done tile replacement in our house twice, and I don't think you can. I think you have to break them out first and put in new concrete backer-board. Otherwise any uneven places in your existing floor could cause the new tiles to crack and break.

I install new floor tiles over older existing tiles quite often. As long as the base and tiles are sound this can be done.

If you want to do a tub surround then proper/recommended thing to do it start fresh with concrete board and new tile. If it isn't in your budget or time you can tile over existing wall tiles.

Start by scuffing up the existing tiles then trowel on mastic rated for wet areas. The grout as usual. There are special end tiles that will have a large lip so they cover over the ends of existing tiles so you don't see the double tiles at an exposed edge. You may have to get these from a tile store and no a home improvement store.