Hand selection: When you begin to play, the most important thing is to fold all the junk preflop. Does that QT look good to you on SB against a raiser? Don't do it! Just toss it away.
Fold: Don't be that guy who calls preflop with 89 offsuit and then calls three post-flop bets after you flop a pair of 9s.
Position: Play more hands from BTN or CO. Do not over defend your blinds. Tighten up the more you are away from the BTN.
Aggression: If you have a strong hand, raise raise raise. Don't slowplay unless you have a huge hand. Think at least a flopped set with no obvious draws at least. Don't be that guy who flops top pair, tries to "trap", bets small on the turn, and then calls off the whole stack on the river when he gets check raised, and then calls villain a "donk."
Attention: Pay attention to your opponents. Try to detect patterns. The correct play against one villain isn't necessarily the correct play against another. Use a hand tracker if possible.
As you get more familiar with the game and get better at situation/villain reading, then you can start to mix things up.