There are a number of free banner makers on the internet. The only bad thing is that they put their logo in the corner of your banner. You can at least use them until you learn how to make your own banner with a photo editor.
Ad Designer is one of the easiest banner makers to use. They have some very nice designs.
Pure Silva Banner Maker has some very nice backgrounds and features.
Banner Fans seems to have the same banner maker that Banners Go MLM provides. You can make simple text banners and upload some of your own backgrounds.