Teenager joining a gym? What equipment should I use? Other tips?

I recently turned 15 and i want to change my life a bit by joining a gym. I'm hoping by doing this i'll improve my confidence, increase my BMI (i'm currently 17, which is underweight) by gaining some muscle mass, and also I'd be joining the gym with a friend who I'd like to spend more time with... show more I recently turned 15 and i want to change my life a bit by joining a gym. I'm hoping by doing this i'll improve my confidence, increase my BMI (i'm currently 17, which is underweight) by gaining some muscle mass, and also I'd be joining the gym with a friend who I'd like to spend more time with and get to know better.

So do you have any tips for me?

Also, what gym equipment should I use and what sort of exercises should I be doing to gain muscle mass but try not to loose much fat? As i said, I'm underweight so i don't want to loose weight by joining the gym.
Would using a rowing machine help me? Would it build muscle in my arms and legs? Anywhere else?

Any other tips would be great :)

Thanks :D
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