Most people want to get the "BEST" looking footage uploaded to YouTube. If you follow the YouTube Guidelines for Apple users it looks something like this:
Share -> Export Using Quicktime
DO NOT PUSH TO BEST. I know it seems like you would because you want the "best" but the H.264 encoding actually does a better job if it has a little "work" to do. So set to High or a bit more, but not Best. This will also help the file size not be unbelievably HUGE.
You will want Multi pass on and make sure to turn off Frame Reordering. I would also urge you to try the X.264 codec. (Watch Advanced Video Exporting in Tutorials section if you don't know what I am talking about)
Select the correct size! You are not going take Standard Definition (SD) 4:3 footage and magically make it HD. So if it is not an HD camera then it is SD. That means NTSC 720x480 in the U.S. or PAL 720x576 in Europe.
Upload to Youtube