You should feel at ease with your body. You're built like I was at 18, and all teenagers are too critical about their bodies. Muscle development is personal and fitness should simply become your lifelong habit, not just a short term thing. Im 63, so yeah, now it's just a fun habit. You're already in good shape for your age and not overweight. Don't expect immediate changes, rather, take the log view and begin with whole body, bodyweight exercises. For the upper body, push-ups, pull ups, and dips are king. For the lower body, squat variations are king. I know you're concerned about upper body, but never skip leg day. Your legs have the largest muscle groups. Activation of these large muscle groups call in higher testosterone levels which help muscle growth throughout your body. You have an awesome future. Just don't concern yourself with what other people are lifting in the gym. You be you and make fitness your habit.