Tactics is a great way to improve calculation, but I would also encourage you to learn opening principles and development. There are several ways to learn opening theory, but the best way is to pick one and study the first few lines of play. Don't try to memorize every line, just learn the first few main lines and then put theory into practice. The best way to learn new openings is to familiarize yourself with the moves, and then watch videos, or talk with a coach, chess mentor, or use media. I encourage you to take time to learn the opening theory just enough so that you will play more sharp and precise. The rest is tactics.
I suggest learning 3 openings; one for when you play white, one for when white plays e4 against you, and one for when white plays d4 against you. Then you only have to learn three, which isn't very hard.
I would just focus on completing your development and not dropping pieces. Each time your opponent moves, ask yourself, "Is the piece he moved attacking one of my pieces or did his move expose an attack by one of his other pieces." Before you make a move, ask yourself whether the square you're moving to is attacked or whether by moving the piece you have exposed one of your other pieces to attack. Start there and see if you don't get up to 1200.
Thats a lot of things! Maybe it would be simpler to use a list:
Three "no" answers mean you may be about to make a mistake.