Elo rating
Hi. I read Amateur's Mind by Jeremy Silman and learned a lot about planning and strategy that I didn't know. I feel its helping me progress towards 1500.
Tactics, tactics, endgames and tactics.
In terms of endgames, study king and pawn endgames, opposition, etc. And most importantly, look at rook endgames, particularly king, rook and one pawn vs king and rook. How to play with the side with the pawn and against.
Opening theory is the last thing people tend to study.