This tutorial was created using PSE 8, but can be duplicated with pretty much any version. If I'm wrong.. be gentle :)
1. Open your photo in Photoshop then gaze and love. I love this straight out of the camera shot, but it's just too dark. This little angel needs to shine!
2. Create a new levels adjustment layer by clicking on Layer>New Adjustment Layer>Levels.
3. Click OK when this little screen pops up. It doesn't pop up in every version of Photoshop, so no worries if it doesn't appear.
4. You'll see a series of mountain ranges (okay, not really but that's what it looks like). You'll see three little arrow thingies (that is the official term, people) in black, grey, and white. The black arrow represents the shadows in the photo, the grey represents the midtones, and the white shows the highlights. In a well-exposed photo, the "mountain ranges" should spread evenly across the whole box. Mine obviously doesn't. You can see the ranges fall short of the white arrow. So, we're going to click on the white arrow and slowly drag it to the left. Watch what your photo is looking like. You can also move the grey arrow to the left or right, as well as dragging the black arrow to the right if your shadows need work. Play a little!
5. This is what my Levels "box" looks like now.
6. And what a difference a small levels adjustment can make! I have done no edits to this pic beyond that little adjustment layer. A huge difference, right? Have fun playing with Levels!