Start at the beginning, focus on preflop play, thinking about what hands to open with in what position. There are a number of charts out there that can help with this. Also, if you download a hand equity calculator like Equilab, the software has some built in preflop charts.

You will make the fastest improvement in your game and profitability just by not playing bad and mediocre hands from early position or junk hands from anywhere. While It seems as though it is cheap to just limp in with some weaker hands, doing so is a mistake. It's often only a small mistake on an individual hand. But, what most players fail to realize is that the initial mistake leads to big losses in two different ways. First, it puts them in a bad position later in the hand which leads to bigger mistakes on later streets. Second, because you play every hand preflop, you will be making that mistake very often which will quickly multiply into big losses.

Also, remember that when you are the first one in a pot, you should ALWAYS be coming in for a raise, not limping in. If there are already several limpers in the pot, you can limp in from late position with some hands that can hit the flop hard, like small pocket pairs, suited aces and suited connectors. But, you do not ever want to limp into the pot if you are the first person in.

Dave

@Dap Poker
Excellent advice. Dap is right on point.