This tutorial will teach you how to make a quick and easy starfield background for use in signatures and other art.

Step 1: Create a new document, any size, any resolution
Step 2: Press "D" to set colors to default
Step 3: Press "X" to switch foreground with background color
Step 4: Press "CTRL+Backspace" to fill the Background layer with black
Step 5: Filter->Noise->Add Noise (10, Gaussian, Monochromatic)
Step 6: Press "CTRL+L" to bring up the Levels window
Step 7: Input levels: 0, .25, 100

And you're done! For some added effects, try adding Lens Flares (Filter->Render->Lens Flare) which when you set to a low brightness, look remarkably like bright stars.