I'm a 14 year old girl starting up biking and I have some questions?

It takes a while to get use to the saddle just make sure that it is at the correct height and tilt. Cycling shorts will help a lot. You want to do an easy 10 min warm up ride and the start spinning at 70-90 rpn's all the time in a gear that is easy. Pedaling slow and mashing on the pedals is hard on the legs. You don't need anything except water on rides less then 90 min. A protein shake after a hard ride will help build muscles. Cycling is a cardiovascular exercise that is great for your heart and lungs, It will tone your legs and butt but they will never get big. You want to lose weight eat less it's very hard to out ride a bad diet. If you plan a long ride load up on carbs the day before so you body has a good supply of glycogen. Take along an apple and banana and some Good & Plenty to much on as you ride.