After my initial weight loss, I still didn't have any definition anywhere. I would lift a little at home, but I really didn't know what I was doing. When I joined a gym, I lifted with the machines. I felt stronger, but I usually just did my favorite lifts, not the ones that I really needed to do.
Squats and lunges have made the most difference in my shape. This summer, I signed up for training with a trainer/bodybuilder. The moves she showed me have made a huge difference in my upper body. I have shoulders and biceps now. She told me I should always wear tank tops. She also had me doing assisted pull-ups. That is a move I figured I could never do. Maybe I will get to unassisted pull-ups with time and maybe I won't, but I am getting stronger.
Before training, I was lifting for 20 minutes. She told me if I really wanted to build muscles, then I should shoot for 40 minutes. Each lift should be controlled and breathing on the exertion is a must. Taking breaks (1 min.) between moves is also necessary. I think the breathing and contracting has been the biggest change. It doesn't matter the amount you lift as long as you have correct form. Add weight as you get stronger. Learn to do it correctly first.