Help with dvds!!!!?

I have downloaded youtube videos and I try to burn them to a dvd rewritable disc and they won't burn and I tru to convert their format and it still doesnt work what should I do? I've even tried burnin it in Windows Media 11 and it still doesn't work. Im using nero so can someone post the steps in what I have to do or email me at rayvitald@yahoo.com or by messenger: gokuse