Here's a fast tutorial that you can use to make a glitter effect on any image. There are plenty of different ways to do this, and each will yield different results. This one's pretty simple so it's a good one if you're just starting. Off we go...
Open up your source image. In this case, we'll use the always rockin' Steve Tyler of Aerosmith.
Step 1: Select a brush with a wide pixel spread, and increase its size to something that makes your sparkles look exactly how you want them to look. I used a dry brush.
Step 2: On a new layer, paint some dots over the image in a color that you can see. The color doesn't matter at this point, though, since we'll be changing it in the layer style interface.
Step 3: After you're content with the sparkle placement, double-click the layer to get into the Layer Style interface. Here, play around with gradient fills and opacities to get the effect that you want.
I chose one of the standard color gradients, with a blend method of "Screen". Adjusting the layer properties rather than painting the sparkles by hand will save you time, and let you play around with different looks quickly and easily.
Step 4: Once the colors work for you, select the custom shape tool, as seen below.
Pick a star shape and draw it out over the image. It will become its own layer. Now, copy it and resize the shape all over your image until you have the stars in the spots you want, and adjust its layer properties just how you did in Step 3.
Eventually, your image will look something like this:
Play around with settings to get this look how you like it. Have fun!