To make scan lines all you need to do is create a new canvas in Photoshop with a size of 3px x 3px. Like so.
Once you have done that , zoom in as far as you can. Then grab your pencil tool and draw three squares in a diagonal accross the canvas. Like so.
Now go to edit --> define pattern, give it a name and click ok. Then open up the photo of graphic you wish to add the scanline too.
Now sometimes scan lines are a bit to strong on certains colours so I suggest you make a new layer in your project and then hold down shift and backspace on you keyboard. Now a small pop-up window should appear, choose pattern from the drop down and then underneath look to try and find the pattern you made in the library. Then hit ok. Now you should have scan lines. If you have any problems post here again and I will help. Now with your scanlines on a layer of their own you can adjust the opacity to what looks to best.
Good Luck!