Expand your text using the Object menu, then through the Windows menu go down the Pathfinder palette, It's alphabetical arranged, then with the Pathfinder palette displayed make sure that the text is to the front, you may have to ungroup the expanded text ----> click on Minus Front. ----> you might need to do this for each of the expanded letters, it depends on which vers. of AI you have.

You can also use the Shape Builder Tool to do the same job (Shift+M) then Alt+click the area you want to remove. Again each must e a shape so you have to Expand the text and ungroup.

Do a search for "cutting a shape using Illustrator" would help you understand the tools for as there is many ways that you can do this.