You can take the exam after one year, but it is best to study for two years. After all, you are 15. The exam fee LS has already been mentioned, and I have finished the exam at 10, so I'm not sure about the price. And you don't need a level one exam. If you have enough ability, you can skip one level. But you'd better not jump too much, just level 2 and level 3, otherwise your study will be tight and your foundation will be weak.