How to get better if I'm too weak

So currently I started work out. I wasn't an active person and I am always the worst students in PE classes. I can't do a single push up. I tried some weight machines, I used the smallest amount of weight but some of those machines still really heavy for me. What exercises should I do to get better?
(I'm 15, I don't know many things about workout so please don't hate me if I say somethings wrong.)

by mariaooe via Bodybuilding.com Forums - Female Bodybuilding