Hey, I'm 16 and I wasn't exactly in your situation at 14 but still doing more or less what you should.. I was a fat kid so I've been losing weight for about 3 years and now I'm not fat I want to build some muscle.
It's all down to the kitchen really. I think 14 is a little young for weight training but i've seen some people in my school gym who are your age and building up.
You're going to need a lot of protein in your diet if you're weight training anyway. Weight training breaks the tissue of your muscle so it has to repair which makes them grow. Protein makes them repair and grow faster so you can simply get back in the gym quicker..
Don't go to the gym every day of the week.. 3-4 days a week max. Your muscles grow when you're resting, not when you're in the gym - that's when you're breaking them down.
Does your school have a gym? If not, you can wait until you have the money, maybe ask for some equipment for christmas / your birthday.
For now you can use your own body weight to gain muscle, push ups, crunches, pull ups, dips.. have a look on google or youtube on how to build muscle with no equipment or something like that.
I'm sure someone more experienced can tell you a lot more but hope i helped