It depends on your version of Photoshop I suspect, but in CS6 if you have the Character dialogue open (go to Window along the top menu bar and it should be in the list), there are various elements of a text box that you can modify, including spacing between individual letters. In CS6, the symbol in this dialogue box you'll be looking for is a VA with a two-headed arrow underneath, and it is set to 0 by default. There is a T also with a two-headed arrow, but that stretches or squishes the text.