How do I burn a dvd from an iMovie?

Use iDVD.

Apple no longer sells it. Buy any version of iLife 11, 09, or 08 from your favourite software vendor and install just the iDVD app.

Amazon: http://www.amazon.com/Apple-MC623Z-A-iLife-VERSION/dp/B003XKRZES/

Good info here on both iDVD and a free option.

https://discussions.apple.com/docs/DOC-3711

If you have another Mac with a fairly recent version of iDVD you already own, you can copy that app to your new Mac. It will work.