Guys, Tips for weight (muscle) gain?

For now I just have myself and a 10 pound dumbbell..
I need tips! I'm 15 years old, 5'9 135LBS..
I'm playing safety for my JV Football team..
Any suggestions on what I should do to build some muscle? Is my weight and height okay for my age? Any help is appreciated.. Thanks!
Guys, Tips for weight (muscle) gain?
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