Are you doing classes or your own routines? If you want to build muscle you need to use heavier weights and do lower reps til failure (which im sure you know). free weights are better but form is very important and if you dont know what you are doing you could injure yourself.
My advice would be (if you can afford it) get a personal trainer for a few sessions. They will talk through your goals, assess your current strength and fitness, and give you a routine to start with. Once you have the hang of it you wont need them for a little while at least. eventually you will need to vary the routine and up the weights to progress further. but by then you should have more of an idea of what you are doing. and just ask loads of questions, of trainers, experienced people in the gym etc.
its also very hard to motivate and push yourself as well. if you can find a training partner thats a big help. or join a couple of classes!