Re: I am really weak
It's ok. That's good. Although I suggest that you learn something from a coach at school or a personal trainer at a YMCA - you could lift the weight the wrong way (VERY easy to do - swing during curls, bend over during push-downs [which is what you described - pulling down from really high] etc). If you want to be careful, just like you said before, make sure you're doing it right. A trainer is the best way to go on this.
If you lift the weight wrong, you could pull a muscle, drop the weight, work the wrong muscles, ache severely, not gain muscle as fast and injure yourself in some other way. Usually, not just one problem happens