It is quite simple really,
In photoshop cs2 or 3 go "Image" > adjustments > levels and the levels dialogue box will open. Or go Ctrl + L.
You will see a histogram with the white slider bottom right and the black slider bottom left. A general rule of thumb would be to drag them in to the point where the "uphill" starts.
If your histogram is clipped - the "hills" extend beyond the sides of the graph, then just leave them at the extremes and play with the mid-slider - the grey one bottom centre.
Another way is to press and hold Alt while you bring in the white and grey sliders. One at a time of course.
As you bring in the white slider from the right you will see spots of different colours at first and they will grow and intensify into white signals.
Your subject matter will dictate how far you will bring them in so have a ball playing around.
The same is true for the black slider brought in from the left. Press and hold Alt while sliding it in and then decide how far you want to bring it.
Now fine tune with the mid-slider.
Rather rudimentary explanation but hopefully sort of understandable and once you know levels you will love it.
First prize, of course, is getting the exposure right when you click the shutter
HTH!
