You have to change your grip when you are in the bunker near the green. Choke way down on the club, perhaps 6 to 8 inches lower than where you normally hold the club. This will give you greater control and feel for the ball.
Normally, you stand with your feet about shoulder length apart. Narrow your stance significantly and make sure the ball is about two or three ball lengths forward in your stance. Dig in with your feet to get more traction. It might look and feel a bit strange, but this will help your get the ball up in the air.
This is the most difficult part for most new golfers. To get the ball out of the greenside bunker, you do not hit directly on the ball. Instead, you hit about 1 inch behind the ball. This will cause the sand to explode into the ball and carry it up and over the lip of the trap. If you hit the ball directly, you could fly it far over the green.