Song writing question?

I'm 15 years old (male). I love to play piano and sing, but I want to start writing. I have tried time and time again, but I'm not really liking what I write. I can get inspired by my favorite bands/musicians (the fray, coldplay, sara bareilles, james morrison) but I cannot write lyrics without sounding somewhat alike to the music that I admire. Every time I write a nice piano riff or some good sounding lyrics, they sound like that of something that has already been played on the radio. I want to change that, but how can I approach that? I also want to start writing lyrics, but every time I write them they sound very boring and childish, which I am still very young, but I would like to come off as more poetic in a song with more meaning, but it seems so impossible. I want to be able to write as a way of venting, but these problems I just listed are really preventing me from doing so. I don't know where to start off when writing...melody, lyrics, piano...lyrics, piano, melody...not sure! Any help is appreciated. I want to write meaningful music, nothing like something the Jonas Brothers or Miley Cyrus would write.