You will need to have the program iDVD. If you do not have it, it is readily available on Amazon as part of iLife 09 or iLife 11. Yes, there are alternatives to iDVD but they have nowhere near the ease of use or power of iDVD.
Go to File>Share>File, choose the size (probably should choose either SD or Large) then click Next, give it a name, then click Save.
( If a movie is not selected, the Share button will be greyed out. Select a movie, then click Share. )
Go to iDVD. Go to File>Import>Video and select the file you just created. It will bring the video into your iDVD project.
For better quality, I normally use iMovie HD 06. However you can use iMovie 13 as well.