Answer

Hello,
You can tile over existing tile. First you must make sure existing tile is secure to wall with no chips or cracks. Remember, any new surface will be as good as the substrate it is attached to.
Go to your local tile supply store, Lowes, Home Depot, Bath and Kitchen Supply, etc and ask if they carry adhesive made for tiling over existing tile. If they don't or you can't find it then don't proceed without it. If they have it, then pick up a tile scorer, and the adhesive. Use a degreaser to clean the wall and thourghly rinse and let dry. Score your existing tiles vertically and horizontally to create a bonding surface.( UNLESS THE DIRECTIONS SAY YOU DON'T HAVE TO SCORE THE EXISTING TILE. Some adhesives require scoring)
When applying new tile stagger the joint lines. Do not line up new joint lines with old.
There are also self stick tiles that can be applied over existing tiles.
Remember, when you tile over existing tile the edges along the door, window, and molding will be sticking out the thickness of the new tile and you may have to modify these areas. Also, any electrical outlets will have to be addressed as well.
One final point, do not use a heavy wall tile on top of the existing tile. If you have 4x4 tile now, use 4x4 when tiling over.
Hope This Helps -Hammerhead-