Convert it to an AVI with XviD video and MP3 audio or an MP4 with x264 video and AAC audio using any one of a number of programs. YouTube accepts both of those formats. As for the best possible quality, anything uploaded by you gets reencoded by them and severely degraded.
You might try AutoGK. I think it accepts DV AVI input. It'll output XviD and MP3, if that's OK with you.
YouTube has its own guides for this sort of thing:
http://support.google.com/youtube/bin/static.py?hl=en&guide=1728585&page=guide.cs