If you just want them in a way that only iPhoto can read -- truly a "backup", just select the photos you want to backup and use the "burn" button in iPhoto. These will not be able to be read by other programs nor PC's, but are a good backup for your own computer.
If you want to copy them to a DVD and perhaps let others use them, then select the photos you want, export them to a folder on your desktop, and then using Toast or disk utility or such burn them to a DVD. I believe if you insert a DVD, once it shows on your desktop you can drag the folder with the photos to it and it should burn just fine.