help a n00b

Just getting started with photoshop, I want to create a grid like layer to go over a piece I'm working on... how would i go about this? Surely I dont have to draw each line individually..

Cheers

Ok, here's what you do:
1. Click the rectangular marquee tool.j
2. Change the style to fixed size and enter x (20 works good) for both the height and the width.
3. Create a new layer and fill the square with black.
4. With the square still selected, go to Select>Modify>Contract and enter 1 pixel.
5. Delete the inside of the square.
6. Now Ctrl>click the layer with the square outline and go to Edit>Define Pattern.
7. Then if you want to fill something with the pattern, go to Edit>Fill and select the pattern you just made.

Hope that helps you.

Mad,
now one last question... how do i get it so that the grid fades away over one section of the image but is visible in the other?

You'll need to add a layer mask to it. To do that, just click the button that looks like a white circle surrounded by a shaded square on the layer palette. You see, on the mask everywhere you paint black will become invisible and everywhere that is painted white will become visible. What you need to do is apply a black to white gradient to the mask so that the visible portion will gradually fade away. That's all there is to it.