x_thaone - as far as technique goes there are two really important things to think about.
1. Pull your shoulder blades back when bench pressing. Many people let their shoulders round forward when pressing.
This will increase stress in the shoulder joint and can cause pain. The stress will usually cause problems with your biceps tendon (which crosses the shoulder joint) or rotator cuff.
A reason that many peoples shoulders round forward during the bench press is that they neglect training the muscles in their back. A good bench presser has a strong back. It is safe to say that your back is not strong enough. (that statement is always true)
A good rule of thumb here is to do more sets and reps with back exercises than with pressing exercises.
So shoulders back, chest out is good.
2. Elbow placement can cause more stress in the shoulder joint. If you let your elbows flair out it will increase stress on the biceps tendon and rotator cuff.
A good rule of thumb is to keep your elbows under the bar and touch the bar lower on your chest. This will decrease shoulder rotation.
3. If you are going to start doing rotator cuff exercises make sure that you are focusing on training your external rotators. These are muscles that help stabilize the shoulder joint during pressing exercises.
The best advice would be to find someone who can watch your exercise form and give feedback.
because before you turn 18 you shouldn't lift more than your own body weight.
There is nothing wrong with lifting heavy at younger ages as long as you learn proper form for the exercises you are using first.
Only exercises done with improper form will cause injury or damage to joints.