Don't tighten the clamp too tight or to slack, you want it enough so that when you tap the centre of the hole inside the clamp with the correct hammer, you won't be able to feel the vibration on the other side of the tile.
You want to start off with very very light taps right in the centre of the hole in the same place, on ceramics or porcelain-glazed tiles you'll start to break through the surface in time, on marble you'll start to see a small indentation pretty quickly.
Once you've started to chip away, keep tapping lightly in the centre, make the indentation bigger. As you get deeper you can hammer around the small indentation, but don't go near the edge of the hole in the clamp until you have worked your way right through the thickness of the tile in the centre.
Once you've created a small enough hole to see light through, you can start to move away from the centre towards the edge of the clamp hole.
The aim is to chip away and not actuall hammer the tile. So you're just using the point on the hammer tip to gouge a hole into the tile. You can neaten it up afterwards with the hammer around the edge, but some prefer using your tile nippers at that stage to creat square holes or large neat round holes.
Let us know how you get on.