You need to know what hand you have and what to do with it.  The lowest hand you can win with is a pair.  A pair is when you have the same two numbered cards.  This hand is one hand of 9 hands.
The second worst hand you can get is the two pair.  It is when you get two cards the same and another two cards the same to become 2 pair.  This hand is very common in poker.
The third worst hand in poker is the three of a kind.  Three of a kind is when you have 3 of the same numbered cards in your hand.  This hand is another one that is most likely to get during the game of poker.
A straight is the fourth hand to get in poker.  It is when 5 out of the 7 cards you can use are in an order or a row.  A straight is really not that hard to get in poker.  I have got a straight a lot of times.
A flush is the middle best hand to get in poker.  It is when 5 out of the 7 cards you are using have 5 suites the same.  The hand is somewhat rare to get during the game.  The past 4 hands will not beat this hand.  I have won money lots of times playing this hand throughout the game.  Poker is just a game of luck.  Some skill though.
This hand is called a full house.  It is when 3 out of the 7 cards you are using are the same.  Also, 2 out of the 7 cards are the same too.  5 cards total make up the full house.  The previous hands won't beat the full house.  The full house is one the hardest hands to get.  I have won pots off this hand.  It is very hard to keep a straight face when you have this hand.
This hand is a four of a kind.  It is almost impossible to get.  It is when 4 out of the 7 cards you are playing  with are the same.  Like all four 4's or all four 8's.  I have got this hand once or twice.  All the previous hands won't beat this hand at all.  
This is a straight flush.  It is a very hard hand to get.  It is when 5 out of the 7 cards you are using are in a row and are the same suite too.  It is really not considered a hand because it is really hard to get.
This is a royal flush.  A royal flush is when the ten, jack, queen, king, and the ace are in order and the suites all match.  This hand is impossible to get.  It probably has been done once or twice in all the time the game of poker has been played.  This hand will beat any hand in the game no matter what.
This hand is a card high.  It is when you don't have anything and you want to put out your highest card.  Normally you want to fold before this hand, but if you are all in, you don't have a choice.