Realize this will take time. Are you in for the long haul? It is seven months until hunting season.
I am a HUGE proponent of ONE arrow practice at short distances to build shooting muscles.
Start by stretching really good. Place a target up off the ground so you are shooting level. Back away about five or ten yards. Shoot a single arrow. At this point it matters little where it goes in relation to "good shooting". BE PATIENT! Walk slowly to the target, pull 'er out, walk slowly back and shoot again. Shoot as many times as you can. STOP. Go inside and rest. Come back in fifteen or twenty minutes and do it again. If you can do this twice a day, even better. Do this for a couple weeks. CONCENTRATE ON USING YOUR BACK MUSCLES!!!
The next step is to add an exercise of beginning your shooting by coming to full draw, holding it for a three count and then SLOWLY letting down. Now shoot as before. Proceed with your one arrow practice. Do this a couple weeks.
As young as you are, I bet you will be smacking a half dozen arrows pretty well about anytime you want to, from about any distance. BUT, GIVE IT TWO MONTHS BEFORE YOU START TRYING TO DO MULTIPLES.
Trust me on this one. It is profoundly tedious, I know--so is weight lifting. The one arrow practice gives you time to think about form, push/pull, draw, aim, anchor, release...or is it aim, draw, anchor, release, hmmmm. You can shoot more times without getting worn out and whiffing arrows all over the place.
I live in the frozen tundra of eastern Idaho. This is how I practice all winter long. When you can do this for a half hour in a frozen garage, you are getting close.
So, are you in for the long haul?