A key concept to master in poker is pot odds, which let you calculate if it's a good bet to stay in a hand or not.
Basically, the size of the pot matters when you decide whether or not to call a bet to stay in a hand. To put it simply: If there was $100 in the pot and the bet was $1, it would be silly not to stay in: You're getting a chance to win $100 for only $1, or 100:1 odds. You'd only have to win once every hundred times for it to be worth it. On the other hand, if there was $1 in the pot and someone bet $100, it would be silly to call that bet -- to put $100 in the pot to win just $1 -- your odds are 1:100. You'd have to have the absolute stone cold nuts to do that.
So to figure out what pot odds you're getting, its the ratio of the size of the pot to the size of the bet. A $10 bet into a $50 pot is 50:10 or 5:1. A $5 bet into a $75 pot is 75:5 which reduces to 15:1. It helps to remember your fractions!
Once you've got the pot odds, the next step is to determine whether or not the pot is giving you the right odds to stay in. You do this by calculating your outs
and the odds of hitting your hand and winning the pot:
Once you figure out the odds of hitting your hand, you can compare it to the pot odds you're getting. For instance, if you're getting 5:1 pot odds ($20 into a $100 pot for example) you want the odds of hitting your hand to be 5:1 or better. If you have eight outs, for instance, your odds are roughly 2:1 of hitting your hand so you would definitely make that bet. On the other hand, if you only had two outs, the odds of hitting your hand is roughly 11:1 so it's a bad bet to make.
If you follow the rules of pot odds over the long term, it will help make you a winning player.