Ok, very simple tutorial showing you how to make T.V. Scanlines.

First, create a new document that 1 x 2 pixels (1 pixel wide, 2 pixels tall). Then zoom in all the way so you can see it (1600%).
Select the pencil tool and click once on the upper portion of the document. You should have something like this:

Next, go to Edit > Define Patter... Call the pattern whatever you would like (Most preferably scanlines or something like that).

Now open the picture you would like to use the scanlines on. Go to Edit >Fill and then select the scanline patter that you created.
I recommend these settings:

3 simple steps and you are done. If you would like to add more variety to your scanlines you can use a different color
instead of just black.