You'll need to use your brush tool to get that sparkle or star effect. I'm really not that great at giving instructions but here goes:
1. Choose the brush tool from your tool palette
2. At the top of the screen click on the brush menu and if you don't already have it selected, choose assorted brushes
3. Scroll down the menu until you see the star or sparkle brush you want (there's a few to choose from). You may need to adjust the size of the brush using the Master Diameter slider.
4. Make sure your color selection is on white (can press "d" to get to default black & white then "x" to make white your dominant color)
5. Airbrush sparkles to your heart's content! I suggest making a layer just for your sparkles so you can move them around and place it exactly where you want it.
Hope this helps - let me know if you need more help