Lanky is right. Reading is the best workout a 14 year old can do. Look into nutrition too. If you focus on nutrition. [Lets back up] You are only 14 and no one expects you to go crazy with nutrition. Your a kid eat ice cream and enjoy it while you can. But besides that, you can do little things like make sure you eat breakfast. Eggs are your friend. Eat good during lunch. Remember a key but just 1 of the components of building muscle is protein (those are your meats, beans, nuts and dairy foods). But once you start to get a good diet down [this may be more in the future] you can really start working out.
Does your school have a weightroom?
A mistake you can make is one I see a lot but you could easily avoid it. Kids want to workout but they are too afraid to do the important stuff around other people in their school and sit around on machines for a few months until they start experimenting with other weights. Trust me I've been there.
I would say pushups and pullups at your age but I have no clue if there is any truth to that, those workouts being less stressful for you. LankyLeftys video in this forum is a good leg workout. Hard as can be, but you have to be mentally strong to do it. When you do the full version you have to believe in yourself. Remember heart and desire will make you bigger. That is step 1, step 2 and beyond is doing the right exercises and eating the right stuff.
I got a good one, I think. It is a yoga ball chest press. I used to do this one a lot and like it, a good home workou to do.
I'm not sure how much you should be working out. I don't think it will stunt your growth but I'm not sure what it could have to do with your body while its growing. I can't tell you much sorry.