I'm doing the beginner's 60 day course and introducing…

I'm doing the beginner's 60 day course and introducing myself is the next thing on the list. So, I am 41 years old, 6'3" and 242 lbs. have 5 kids, married, about 20-25 pounds overweight, and looking to gain muscle, lose fat (around the stomach and chin), and get stronger with better mobility. I have read that after about 40 years old a person generally loses 1% of mucle mass and 1% of bone density each year, so I'm trying for that not to happen to me as I age. I also do not have the best posture in my neck, I slouch quite a bit.