How can i gain speed on my fastball?

Im 14 years old and im not very tall so i will still grow but how can i gain speed on my fastball. Last season i threw only about 55 mph but when i started pitching this year i threw right around 70 but how can i do something by myself to gain speed? Also how can i throw a better curveball and changeup i had a really good slider at around 65 mph and that was my main pitch but i didnt really throw a curveball or a change, i had one but they werent as good as i wanted them to so how can i get more speed on my fastball and get a better curve and change up?