If you're in Photoshop, grab the Rectangular Marquee tool. In the top left corner click and hold then drag to the bottom right and let go of the button when you get to the place you want. It doesn't matter what layer you are on at this point (not really). While the selection is still active, click on Image - Crop, then deselect (ctrl-d). This crops the whole image to the selection. If you want more precise control, then drag out some guides before using the rectangular marquee tool (that's what I do). Normally this process works on the layer that you are on, deleting everything outside of the selection...which is good. So if you had a lot of layers then you would have to do this on every layer. However, if you flatten the image then you only have to do it once.