Although the catcher must posess a number of skills to be a complete player, the fact is the catcher spends most of the game receiving the ball from the pitcher. Many young catchers don't realize the impact of what they do behind the plate and the affect that is has on the pitcher and the umpire. You will have an affect on the pitchers ability to be effective. Be determined to make that influence a positive one.
As a catcher you will have two basic positions from which you will receive the ball.
With no base runners we want to get in a comfortable position where you will be squatting behind home plate. This is a position that you will spend a lot of time in, so make sure it's comfortable. Since you are catching the ball with your left hand, you will want to position your feet so your left foot is slightly ahead of your right foot (just an inch or two). This will shift your body slightly to allow your left arm freedom to move without exposing the side of your body to being hit in an unprotected area by a foul tip.
The following is a good video on the correct positioning.
With runners on base we want to adjust are squat to not only be in position to throw quickly but also to block a pitch in the dirt. The following is another video on the proper position for receiving the pitch with runners on base.
Your catching arm should be slightly bent at the elbow. Make sure you don't get your elbow positioned inside your left knee. This will inhibit your ability to move your arm to catch the ball.