I want to do the three peaks challenge?

I want to do three peaks for concern universal charity. I have to raise around 350 pounds though! I don't know how to raise this much though - has anyone done this challenge and how did you fundraise for it. Also I am a 14 year old girl- I will be 15 when I do it. Is this allowed. I do duke of edinburgh and I am fit .Can someone please help me because I really want to do this and do you think it is a good idea?