Converting videos taken by D5100 for a Youtube upload
Amazingly, the best place to start with a question like this is Youtube. Believe it or not they have help file that explain the process with quite a bit of detail:
https://support.google.com/youtube/bin/answer.py?hl=en&amp;answer=1722171&amp;topic=2888648&amp;ctx=topic

On Youtube itself many others have offered up video tutorials on how to do it:
Youtube.com search

I would take the recommendations that YouTube has on the help pages, and simply apply those settings to your favorite video software, it could be something like Adobe Premiere Elements which is a very common video editor, but many others exist.