There is only allowed to be one soccer goalie (AKA goalkeeper, and often times just "keep" or "keeper" for short) on a team.
You will always use this position on your team.
The keeper is also probably the most important player on the team.
This, obviously, is because he is allowed to handle the ball as long as he/she is within the penalty area. If he is outside the penalty box and he/she touches the ball with his hand; it will be called a hand ball.
The goalkeeper will almost never leave the penalty box.
On offense and defense his job is to protect the goal at all times.
When playing soccer, you will always want to find the best keeper that you can. This player can make a world of difference.
He should be agile and quick with his hands. He should also have no fear of the ball, oncoming players, or diving on the ground.
In other words, the more reckless a player is with his body, the more likely that he will make a good goalkeeper.
This is not the only quality that a good keeper needs. There are many talents that you need to have when you are the keeper, and you are under a lot of pressure during the game.
When playing goalie, communication with the defense is imperative.
However, it is a good sign when the keeper can take a bit of a beating.
A lot of beginning goalies will stand right on the goal line. This is not a good idea.
A keeper should usually stand at least a few yards out from the goal so that the angle will cause a smaller shooting range for opposing offense as shown to the right (bad position above and good position below).