How to get quicker behind the dish?

Practice, practice, practice.  You could have a partner throw you the ball to block, but make sure he mixes them up.  If every pitch is a block it won't help. 

You need lots of practice to judge the trajectory of the ball and whether it should be blocked or not.  It is the hardest thing to do.  Catch a lot of bullpens.  But don't just catch the ball and throw it back.  Work on receiving, blocking, and footwork to 2b.  Good luck. 

Hey RC, welcome.

There are some pretty good on line resources with drills, etc. You might check out the New England Catching Camp and see what they have on line. 

You are at a very good age to be working on your physical conditioning. The stronger your legs and abs are, the easier it will be to get your body to do what you want it to do. 

My son is a very good blocking catcher, and, like you, he didn't start to catch until 7th grade. First, he learned proper blocking form, which should be your number one priority. You can practice your form by yourself, if necessary. Have someone throw balls in the dirt to you, one after another. My son jokes that one reason he can block so well now is that he caught a lot of really lousy pitchers early on. In other words, practice, practice, practice. 

I would say the catch or block decision making is something he has continuously fine tuned over the years. He catches more low pitches than he did when he was your age. mostly because his high school coach pushed him to do it. I would say he had to learn to trust his ability to beat the ball to the spot with his glove.

Good luck!