You are going to have to compress the dvd to get it to fit. This entails either cutting out extras (deleted scenes, extra audio streams, etc.), reencoding the video stream to lower bitrate, or both. Now there are a few possibilities to make your DVD fit on a single layer DVD.
You can use a program like DVDShrink. This is a good program because it is quick and easy. The quality is pretty good.
You can also go to a place like DVDRHelp.com and read their guides on how to do the same thing, but it may take a little more work. Now the pro to this is that the quality should be better. Also, DVDRHelp has guides for DVDShrink, so visit it anyway.