Doubleclick the Background layer in the layer palette so as to promote it to a layer. Then use the magic wand on the image to select the white background. Then delete the white by pressing the backspace key or by selecting "Clear" from within the menus (Edit>Clear I think). Deselect by pressing Ctrl-D. To preserve the transparency you will need to save in a format that supports it such as tif, png or gif. For some of these, saving the transparency is optional. The jpg format does not support transparency.