The Image before improving the contrast with levels.

The image after improving the contrast with levels

Go to Color > Levels.
Some explanation about what you see in the Levels window: The graphic you see here is called the histogram. The histogram is a visual representation of the tonal range of the image. With the image I've used here you see that the histogram is empty at the left and at the right. This means that this image hasn't got any black (shadows) or white (highlights) pixels. And that is the reason why the image as to little contrast

Underneath the histogram there are three triangles. A black at the left, gray in the middle and white at the right.
All you have to do is drag the black point and/or the white point inward so the histogram contains the total image.

The contrast has become much better.

If the result isn't good enough you can move the gray triangle. By doing this you'll compress the shadow or highlight area's of the image. Giving the image more or less contrast. With the example image I've moved the gray triangle a bit to the right to get some more contrast.

And this is the result.