Is it okay to build muscle only in the upper body? What's the best and fastest way to train at the gym? Experts into!

No, unless you only want to grow not much, long muscles, need nerve impulses, only one place to practice is easy to encounter bottlenecks, aerobic exercise is helpful for muscle growth, it is recommended that at least 40 minutes a week, running or other. Leg muscles are very large muscle groups, its practice is very helpful to improve the nerve impulses, naturally bodybuilding essential exercise parts! Exercise methods 2 floor is very detailed, leg exercise has deep squatting, etc., more to the forum, posting bar around, which has a lot of practice methods, some have picture tutorials, such as Baidu bodybuilding muscle and other postings. Gym generally have coach bar, you can ask, there are other people practicing, more exchanges. Find a few companions, but also very important, compete with each other, encourage each other, beyond themselves, to stick down are very useful. This is also the difference between home and the gym, at home, unless extremely strong motivation, super strong perseverance, otherwise, stick down very few. I hope this helps you, and I wish you success!