i can't think about what i want a song sound like. it just comes to me, i'll hear a riff in my head and transpose it onto my guitar and then i'll work with that. i've been playing for like 5-6 years and i've always wanted to make up my own music but i always tried too hard and then i just started playing what sounded cool in my head and it seems to work for me. but everybody is different in their own way.