"Position then submission"?
I know that this may sound rude, but you can submit a guy from anywhere. From every position in BJJ there is an escape and a submission, that's just the way it is.
Really, if you are in a mount I suggest a straight choke, put your forearm on his throat and just lean your weight forward so that he starts to choke. In gi grappling I actually grab the inside of his collar, which makes it alot harder to get my hand and my weight off of him.
The other really popular ones from this seem to be the keylock, which can be kind of complicated for a beginner, or you can swing around one of your opponent's arms and into a straight armbar.