The first step in working with layers is creating a new layer. Most photo editing programs use a layer menu option (such as the one seen here from Adobe Photoshop Elements 6). As you can see from the image, there are several options for types of layers to accomplish specific tasks.
The basic layer type is simply a blank layer placed on top of the current layer. This type of layer can be either transparent or opaque. If you have several layers open already you can chose "layer from background" to create a new layer from the original background layer. You can also use "layer via copy" to make a new layer from items selected on the current layer.
Fill layers are a quick way to create a layer filled with a solid color, gradient, or pattern. This type of layer will most often be used in creating images from scratch rather than photography editing.
Adjustment layers are layers used to adjust characteristics of the background (original) image. Items such as levels, brightness/contrast, hue/saturation can be adjusted using adjustment layers. Options for more advanced changes such as invert to create a negative effect are also available under adjustment layers.