Oh you are that young yet you are already very good!! Umm to be honest, I had a few class about photoshop while I was in college so I learned quite a few tricks. But I think the best advice would be to look at a lot of tutorial (videos, blogs, pictures) here or on other website. You can learn a lot from others! Also you should try to experiment and just have fun to better understand Photoshop! Soon enough you'll develop your own techniques!! If you ever have questions, feel free to ask me!!