The Problem with being on your back, mounted by your opponent is that he controls you. Until you reverse that situation, or escape that mount, no wrist lock or choke technique is going to help you.

You need to learn proper jiu jitsu. That is positional hierarchy. Escaping the mount, passing the guard, shrimping, side control, establishing dominance, and/or escaping. This is all basic JJ.

Shime Waza, (Chokes), joint locks, pins, etc...will not work until you can get a a postiion to do so.