I watched a tiler do this last summer .He got out some rough textured tape and placed it on the tiled wall -over the spots where the holes were needed- marked the position on the tape and used a really thin drill bit to make a pilot hole then he put in the right sized bit and re-drilled the holes for towel rail, loo roll etc etc.
For a first go I would try it on some tile cut offs first.
Depends on the tile but if they are just ceramic then use a masonary drill WITHOUT hammer until you get through the tile, tap the plug into the wall beyond the tile and tighten gently. You can buy special tile drill bits that look like a heart at the end for tougher tiles
You'd use a special tile and glass bit. It looks like a spear-head with a short shaft, and is available in different sizes. Any DIY store will have them. Just ask.
Press the bit firmly into the masking tape on the mark. You'll hear a slight 'crunch' as the bit bites into the glaze on the tile. Start the drill, and quite moderate perssure while drilling. Too much, and you could crack the tile.