Ladykira,
The best way is to create a rectangular brush with a very high spacing. Then find your starting point and for a sraight line, hold shift and draw across the screen. Just remember that the brush will always be angled in the direction you created it, so to make an angled line place it on a new layer and rotate the layer. I've copied the instructions on creating a custom brush from the Photoshop Help file and pasted it below.
This should do the trick. There are about three other ways that i can think of, but this one is the easiest.
Chadoekyll
To create a custom brush shape:
1 Use the rectangle marquee () with Feather set to 0 px to select part of an image to use as a custom brush.
2 Choose Define Brush from the Edit menu.
3 Name the brush and click OK.
4 In the New Brush dialog box, specify Spacing. (See Setting brush options.)
5 To make the brush placement more accurate, make sure that Anti-aliased is selected. (See Setting brush options.)
6 Click OK.
You can also set the spacing and anti-alias options by selecting the brush from the pop-up palette in the options bar and clicking the brush sample. If you make changes, click the New Preset button () to save your changes as a new preset item.
