There are better ones out there but for a starting base I would recommend doing Superman’s. Squeeze at the top and hold a two-five count.
Brown’s suggestion on supermans is a great one for lower and mid back strength. For lats, I’d suggest pull-ups. If you can’t do many, get a rubber band assist until you build your strength up. Straight bar rows, over hand or under hand, are also great. Last recommendation for overall back strength and thickness is deadlifts.