No, submissions will not be accepted without a github repo attached to them. You must share everything you do with the world. If you never learned to share, you probably don't want to join this jam. Post questions related to these concerns here.
Yes, make a github repo and put all of your stuff in there. You don't have to make it public until the contest is over if you're worried about bastids just copying you, but you do have to make it public once you're ready to be rated and see your score.
If you'd like feedback from the judges as you develop, you can add them to your repo: "onexdata", "space-dweeb" and "c-49" are the github accounts that are going to be the judges.