I freely drew a graphic that I scanned and saved in my pictures file. I have Adobe Photoshop CS5 Extended?

and I would like to use it to delete the white background in the graphic. In other words I want a transparent background to the image so that whatever background that I place the image on, the white background will not show and just the color of the background that I choose. I tried doing this and it tells me "Could not use the background eraser because it does not work with alpha channels. I don't know what that is. All I want is a graphic with a transparent background. Easier said than done. I have went into the Photoshop help but it is not giving me the information that I need. Please advise!! Layman terms please!