The diet tips already mentioned are important. I grew up on a farm, where it was easy to build up. @talljasperman is absolutely right about the chores. It may sound like an adult ploy, but it it exactly the truth. Do you know why muscles are attractive to us to begin with? It is because they tell us someone is healthy, and capable, two attributes neccessary for completing a day’s physical tasks. Chores, muscles, they go together.
When I was in the Navy, I was a skinny little woman, 5’3”, 109 lbs as I entered bootcamp. I was muscled, but thin. Bodyfat was too low to calculate.
One part of my job developed my upper body more than any other. Hauling fenders in from over the side. It involved hand over hand motion, lifting a rope with a fender hanging from it which weighed as much as I did, and then flipping it up over a waist high rail. I had to repeat this task dozens of times a day, between other heavy duties.
Remember that whatever you do to build yourself up, to be careful HOW you do it. Lift properly, so you don’t hurt your back. Don’t do too much of the same thing in the same place. Your body needs to change position and routine after a while.
Have you ever watched the Karate Kid movie? He thought he was just doing chores, but each chore was exercising just the right muscles for certain functions.
There is no shortcut to building muscles. You just as well do it by accomplishing something needed. If you don’t have chores at home to work with, look around you. There is always some person who could use help. Maybe a woman who has just had a baby, and her husband is away in the military,or a truck driver or something. Maybe you have an elderly neighbor or two who would be delighted if some youngster offered to take out their trash, or paint her fence, or dash to the corner store and pick up a few things for her. Any one of these tasks would benefit you, and at least one other. BONUS!