Help Building Muscle

I’m 15 years old and am a male. I would like to build muscle mostly in my chest as I have gynocomastia which I hope will go away as I become an adult. It has been very hard for me as I do not take my shirt off in the change room at school I or my gym shirt on and take it off underneath. I use to be overweight then as I became more involved in competitive swimming I became skinny then I quite and didn’t do anything for a year and became overweight once again. Then I joined the wrestling team at my school and became skinny once again. As the wrestling ended and it being now summer I have started Muay Thai which I enjoy a lot and am hoping to do it more very soon. But I feel like I have gained a little more weight again I don’t think I would be considered overweight but still I don’t like it. I got a trainer to help me and we did two sessions and he gave me a short workout routine that only contains only chest excercises however after obtaining that I became a little lazy for like a month and a half so I didn’t do it. It’s currently day two of my goal to get muscular and I worked out my chest both days ( bench press, incline bench etc.) and I did some pushups at night. But when I flex my abs it feels like the sides of my stomach flares out and I’m wondering how I can get rid of that? Also I know it’s not good to workout the same muscle group everyday cause muscles grow when you rest them so I’m wondering what other excercices I can do to just improve my over all upper body (Abs, Shoulde, Back, etc.) I do know I need to workout my legs as well which I already seem to have strong legs maxing out around 75% of machines. Also I do have loose skin and I’m wondering as I build muscle will that go away? And how can I build my chest effectively and make it look nice? Thank you for the help

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