I was riding my new bike and when I was going up a hill on a high gear, and I shifted down toward the top of the hill to make pedaling easier and the chain made a really loud sound and was jerky shift. Should I not shift down in the middle of a hill? I am not sure how or when to shift. Also at the highest or lowest gears, the chain makes a sound, I look down and it clips the front derailer box (dunno the actual name), is this of concern? Could someone give me some advice or good articles to read about shifting?

Thanks!