Trisk

Well I'm assuming you're a guy because you want to start building up muscle...so I can't really give you advice about that. I'm a girl and I *dont* want to build up muscle so I only lift light weights.

But...when I first started going to the gym regularly and stuff, I got a trainer to teach me the basics. If you're really not sure what you should be doing, a trainer is good to have once or twice. It's worth the money. A trainer will teach you about all the equipment, how to use it correctly, what types of lifts are good and probably help you figure out how heavy your weights should be and how many reps you should do. Bad form is never good in the gym and doing things wrong can lead to injury...

Usually, for me though, I just push myself to the limit each time. Don't lift weights that are too heavy for you because usually less weight + more reps are more effective than more weight + less reps. Weights that are too heavy can mess up your form.

I usually alter what I'm doing from day to day\. Some days, my body is just too tired to do, let's say, 50 reps of some lift. I don't get down about it. I just do as many as I can. KNowing your limits is good.
You should pretty much just feel when you're ready to inrease the amount of weights you can handle. If you find yourself able to lift more and more reps of your current weight and it feels to easy than switch....or you could just try the heavy ones after a few weeks and see how it feels.