Not an issue at all, gotta start somewhere. It's probably dated but there's a book called Getting Stronger by Bill Pearl that helped me out when I was a young lad. It's more of an encyclopedia than a cover to cover read, so you may find it useful to keep around as a reference.
Follow the plan laid out by the coach who is willing to help out, and thank him for his time.
Focus on your legs and back when trying to gain weight - they’re the largest muscle groups and the foundation of your strength, especially your legs. Squats, deadlifts and power cleans is what we did back in teh day but that's probably changed.
If you have access to a weighted sled, use it. It is a miserable bitch but my suggestion is to grab a couple teammates and finish your workouts on it. Learn to enjoy the misery together - compete, make it fun, and push that thing all offseason. We didn’t have them when I was younger, but did some workouts with it in my late 20s, and it’s an absolutely grueling workout that mimics what you’ll be doing on the field.
Keep in mind you’re gaining weight for football, not to be a bodybuilder or powerlifter. You need to maintain your speed/mobility as you get bigger. A lot of young players, myself included, get so locked in on the number on the scale that they lose sight of mobility. If you’re going to be on the smaller side, you have to be strong AND quick.
Push yourself hard, stay positive and have fun. Good luck!