First you'll need to download the kit and load the shapes and layer style into Photoshop or Photoshop Elements according to the instructions below. The kit includes Speech Balloons.csh which contains several custom shapes so you don't have to draw your own from scratch. It also includes Speech Balloons.asl, a layer style that you may select when you draw a text balloon.
Instructions for Photoshop
Instructions for Photoshop Elements
Note: Photoshop Elements includes its own set of custom shapes called "Text Balloons" in Elements 1.0 and "Talk Bubbles" in Elements 2.0. You may wish to use these in addition to the custom shapes I've provided in the kit. To access them: Activate the the custom shape tool, then go to the shapes palette in the options bar and click the small arrow in the upper right hand corner of the shapes pallete. A menu will appear with several shape sets to choose from.