You convert a background layer to a normal layer. The one and only optional Background layer does not support Transparency A Background layer transparency is always locked.
There are several way to convert a background to a normal layer. A double click on it in the layers palette. A right click option in the layers palette to convert to smart object. Newer versions of Photoshop will automatically convert it if you click add a layer mask. In the layers palette when the background layer is the target.