Lots of ways to do this really. In general, you would make normal scanlines on a new layer, create a layer mask and create a black to white gradient in the layer mask. Here's a step-by-step.
Creating the scanlines
Obviously, creating the scanlines in the first place is the most difficult task, but it's not too bad. The normal way to do this is to create a pattern. Make a new, transparent document a few pixels wide (even 1 pixel, although if you make it wider it will work faster on larger images). For the height, make it twice the thickness of the scanline (2 pixels for a single pixel scanline). Fill half this document with black, leave the other half transparent, save the document and open the one you want to apply the scanlines to. Create a new layer, Edit > Fill, choose Pattern and pick the scanline document. If you want to change the color, check the 'Preserve Transparency' box and fill it with whatever color you want.
If this technique is a bit daunting, go to
and do a search for 'scanlines' under Photoshop files. Most of the files here are free and well worth a look.
Adding a gradient
Select the layer that has the scanlines. Deselect everything (Select > Deselect) and in the layers palette, click the 'New Layer Mask' button. Click on the mask and draw a black to white gradient. This will make part of the scanlines transparent, but if you need to change the gradient later, you only have to change the mask.
