Welcome to the board weed. Hope your training really takes off. Here are a couple of hints that you might help you in your training:
1. Be consistent. Whatever workout schedule you have chose, make sure to follow it. If you have a choice between doing a workout or hanging with friends, do the workout first. Don't make excuses not to train. If they are real friends they will understand how important this is to you.
2. Don't over train. Your muscle grow
after your workout. They need time to recover, so learn to train hard for about an hour each workout, then relax and give your body a chance to repair itself.
3.
EAT! That is the fuel to get your body to grow. No matter how hard you workout, if you don't eat more kcals than your body uses you won't see the
muscle growth you are looking for. Consume at least 2.2 grams of protein per kg of bodyweight.
4. Be patient. It takes time to change your body. Learn to do the work and wait to see the results. You won't be disappointed.
As to your question about working your lower body it is an absolute must. Because your legs are such a large muscle group working them helps to release extra testosterone into your system. That helps you whole body grow.