Here's a quick and dirty guide to making halftone screens in photoshop, which is great if you want to set up your photoshop files for screen printing!

1) Create your document in grayscale. (Or create it in RGB and then covert it to grayscale.)

2) Using the gradient tool, create a pure black to pure white gradient.

3) Go to mode->bitmap. It'll pull up a mini select screen. Make sure you have "Use: Halftone Screen" selected.

4) Play around with the frequency, angle and shape. I personally like the line and dot screen tone. Select All-> Copy

5) I created a new document in RGB with a transparent background, and pasted in my tone on a separate layer.

5) Using the magic wand tool, (make sure "contiguous" is not clicked!) I deleted all of the white from my screen.

6) Enjoy your screen tone!