Succeeding at poker is a tricky business, but there are a few strategies that can help you win more often.
One of the most common mistakes is to play too much. Poker is a game of luck and skill, but also of opportunity. The best players fold often. Play tight in the early game, and loosen up in the later game, when there are fewer opponents. The good hands will come, and you will be better able to take advantage of them if you don't squander your chips on weak hole cards.
In the same vein, don't be afraid to fold after the flop. Beginners often feel they can't fold after they've invested money in seeing the flop, but at that point your hand is 5/7ths complete. If you don't think you're ahead, you should get out. If you do think you're ahead, make it expensive for the other players to get to the next card (the turn).
Keep an eye on the common cards on the table and be aware of what this could mean for your opponents. If you have two Kings in your hands, but there's an Ace on the table, you are far less likely to win, because any player could beat you by simply having one Ace. If there's a pair on the table, beware of 3-of-a-kind or a Full House. If there are 3 suited cards on the table, your opponent might have a flush.
Chasing a straight or a flush is a good way to lose money. In general, don't stay in the hand after the flop if you have only a potential straight or flush and nothing else. You will lose more often than you will win.
Watch your opponent's behavior. If they seem cautious, you may be able to steal the pot by betting aggressively and forcing them to fold. Don't abuse this strategy, especially if you don't have a strong hand, but it can be useful to bully an opponent when you have a good chip lead.