I'm guessing you want to save the movies into iTunes, not just watch them on your Mac? While it's technically legal to make a back up of your own movies, the movie studios frown upon it, because they want more money; thus, it's not officially supported. It is fairly easy, but somewhat time-consuming (since it takes a lot of cpu time to convert movies from how they're stored on a DVD to today's preferred digital formats.)
First download
VLC. Then download
Handbrake. Open Handbrake and click "Source" and find your DVD. On the right-hand pane, you should see a list of presets. Either "normal" or "high-profile" should be fine; if it's an older DVD, you'll want to use "high-profile," since it has decombing and de-interlacing on by default, which is sometimes necessary for old-DVD's not to get a really annoying windowshade effect from the fact that the source is interlaced. Anyway, just click one of those presets, then hit "Start," let it do it's thing, and when it's done, you should have an mp4 you can load into iTunes.