Ideally, open the original file for the logo (Photoshop? Illustrator?) and hide the background layer. Then save as a transparent image (such as gif or png).
If you don't have the original file, or if you do but the white background is not separate, in Photoshop you can select all instances of a color (e.g. white) with the magic wand tool and then delete them. (To make sure the deleted areas end up transparent, first make sure the layer is not called "Background" in the Layers palette, so rename it first.)
If that's too vague, there are lots of tutorials online for removeing a background in Photoshop, such as http://graphicssoft.about.com/od/photoshop/l/blremovebackg.htm