I'm not entirely sure what you're looking for. Do you want to make a layer semi-transparent? Just change its opacity in the layers palette (if you only have a background layer, double-click it and press ok to convert it to an unlocked layer).
Or maybe you just want to make an image much lighter (or darker), for use as a background, for example? Add a Curves adjustment layer, and drag the bottom-left control point upwards. Note that this default behaviour can be changed, so if it's having the opposite effect to what you want, drag the top-right control downwards instead (and put the other control point back where it was originally).