Hey there big guy. I’ve been training for over a decade and have reached nearly 100lbs over my beginning weight so I think I can help you out. The first thing I’ll tell you may fly right out the window but here goes…go slowly and calmly toward your goal. You should expect to devote at least 3-6 months of good solid nutrition and training before you see the permanent results you are looking for. This is only 50% of the story though, as you MUST make sure that you get proper sleep, avoid bad lifestyle habits such as cigarettes, alcohol and the friends who think it’s cool, junk food (including many poor starches) etc…to achieve your goal. Also it would be a good idea to get a physical with you doc before you hit the heavy weights. You are really only just beginning to be of age where you should implement serious weight training and it is VERY important to get a physical. I had a hernia in the 9th grade that I found only AFTER I had been practicing football. Doc told me that they are sometimes unnoticed until you do something that allows the injury to get noticed…big time. Start with a lower volume and work your tolerance up SLOWLY.
Truth #1: You will get the best arm growth of your life if you do the basic exercises first. I would suggest that you begin with some compound machines like the leg press before jumping right into squats and deadlifts. They are talked about frequently on this site, but these somewhat advanced movements must be taught to you by an experienced coach. And this is OFTEN NOT your football coach. Train the large muscle groups 2x per week. At the very most train 4 days per week. If you cannot devote 4 days EVERY week then back off to 2 or 3 full body sessions per week. In fact, I’ll lay out a very basic set up below on 2 days per week so as not to cramp your social and academic life. If you are interested then you can PM me for more details/help.
2 Day per week: Cycle between each letter like so:
Do 1st set of A1
rest 60-90 seconds
Do 1st set of A2
rest 60-90 seconds
Do 2nd set of A1
rest 60-90 sec.
Do 2nd set of A2
rest 60-90 sec.
Do 1st set of B1
rest 60-90 sec.
Do 1st set of B2
etc…
A1) 2 sets of 10 on the leg press
A2) 2 sets of lying leg curls for 10
B1) 2 sets of a chest press machine or a bench press ONLY if you have a coach/spotter who KNOWS what they are doing. Or you’ll get some free dental work and a bruised sternum (breast bone)
B2) 2 sets of a chest supported rowing machine. The support will put you in proper postural position until you develop the strength to do free weight rows and cable rows LATER.
C1) 2 sets of a shoulder press machine.
YES a machine until you develop some strength and contol and then have a SPOTTER help you with dumbbell presses.
C2) 2 sets of heel raises for your calves.
D1) 1 set of very slow and controlled ab movement such as a crunch for now. There are many better movements that you can learn after you have a foundation.
D2) 1 set of the hyperextension machine. If you are not familiar with this exercise then check the search engine on the site. This strengthens you lower back and will “balance” your ab work from above.
E1) 1 set of a controlled bicep exercise such as a “preacher curl” or a machine. Feel free to do seated dumbbell curls IF you keep you posture good AND you only move at the elbow thus keeping your elbow pointed straight down tot the ground.
AFTER 3 weeks you can add 1 more bicep set for a total of 2
E2) 1 set of a tricep machine. Dips are a great exercise BUT make sure you have someone show you how to execute them properly so as to avoid hurting your shoulders. If nothing else, you can do close grip pushups too. Put you hands on side of two of your school books stacked together and slowly go down until you touch your chest on the books then come back up.
After 3 weeks you can add 1 more tricep set for a total of 2.
OVER. GET OUT OF THE GYM.
Shouldn’t take longer than 35-45 minutes tops.
Resist the urge to do more even if you can. Just because you can doesn’t mean you should. You want to coax the body into results, not make each workout the equivalent of a car accident as far as trauma of mind and body. Do only what is the minimum necessary. Then you will train better and more often later in your lifting career.
The most important thing is to keep good control and form, or you will not stimulate the muscles you want to hit as your “body english” will always shift the stress to your strongest body parts. For now use a 2 seconds up and a 2 seconds down pace on ALL movements to allow for a good controlled contraction.
Sorry so long, but I feel for you big guy. You are looking the right way at the right time. You could have great strength by the time you’re in high school if you are willing to commit now. And one final piece of advice. DONT even consider buying supplements. Eat all the good meat, veggies, fruit, and moderate fat dairy you can comfortably eat without gaining too much in the tummy. A good vitamin and mineral product from a health food store like a Twinlabs Daily one (without iron you’ll get plenty in your meat)is great. Cut out junk fat and carbs such as sweets, soda pop, pizza, and any other thing 13 year olds live on! Just have those on Saturday evenings as a weekend treat. Keep it clean the rest of the time. See you can still have a life for a few hours on Sat evenings. 
And remember: Anything worth doing is worth doing right! Otherwise don’t bother.
take care, bro.
Disc Hoss