You most Know Your Game: If you don’t know your game of choice in and out, front to back, you are at a statistical disadvantage. Many poker players make this mistake and it can end your poker career prematurely. 
Know Your Environment
Online poker is vastly different from casino real poker. They’re practically two different games. Online poker is a whole new experience. You need to learn the difference between the two or it will cost you.
Practice Your Game: If you want to be a good poker player and make money, you have to practice. This means playing as much poker as you can fit. Whether or not you’re playing for money, or just playing to play, you need to keep doing it. As any professional poker player will tell you, poker is a game in which you never cease to learn new things.
Study Your Game: Read as many guides, books, tips, strategies. Poker is a mental game. The more you know, the better off you are.
Know Your Opponent: If you know what kind of player your opponent is, you can know when to bluff, call, raise, fold, trap, etc. It becomes a mental duel. If you know your opponent, you have a huge advantage.