When you are ready to start calculating, count the number of outs you have (number of ways you can make the best hand). Then multiply that number by 2. If you are on the flop, multiply it by 2 again. If it's 34 or more, subtract 1. This is the approximate percentage of times you will win (assuming all of your outs actually let you win the hand).

Next, compare the bet you must call to the total size of the pot. This is called pot odds. You are willing to call bets that are small enough to offer you good odds on the call. That means you have to win the hand often enough that the size of the pot compensates you for the loosing the amount of the bet when you don't make your hand. Specifically, look for this to match the percentage of times you will win (i.e. call $2 into $10 if you will win more than 20%).