Do you have any kind of an athletic background? Sports, martial arts, athletic hobbies of any kind? If you can give us a general idea of what your current fitness level is we can do a better job of helping you get going.
were i you, i’d pick any full body 3x per week program. There are plenty of articles on tn that put out a starting point. Take the time to learn about the basics of muscle building nutrition. You need to eat in a caloric surplus if you want to gain mass. Not all of it is going to be muscle as you will find out if you take it serious. Some folks can gain muscle with little fat gain but I am not one of them. I gained 40lb over the course of 8 months, and now Im cutting and realizing how much of that was fat gain.
Find a program, eat big, do it for like 12 weeks. Then see where you are at and go from there.
if you want to nerd out on the science of it. Check out eric helms Pyramid book. I forgot the exact title but just google eric helms, pyramid and you’ll find it.
Focusing too much on the program (which program is best?) and not on consistency.
Being consistent, but with no focus and effort. No goals. Doing the same weights x sets x reps week after week and not progressively overloading will get you nowhere.
Underestimating the importance of eating.
Being your own worst enemy by worrying about fat gain.
Focus on progressive overload, consistency, and eating. You need to be at least 90% consistent with your training and eating or else nothing will work. Myself and many others wasted a lot of time doing the perfect program but trying to build muscle out of thin air. Muscle is made out of matter. That matter has to come from somewhere.
Also, losing fat is way easier than gaining muscle. If I have to gain 10lbs of fat to put on 5lbs of muscle, I’m taking that deal EZ PZ.