Well, are you writing lyrics or guitar parts? I'm going to talk about writing lyrics, just in case that's what you are doing. What I usually do is start writing lines that seem to make no sense, that rhyme, something nice sounding. After about three or four lines, I realize that what I am writing about I can actually relate to about something that has happened to me or my friends, or something I care about, something in my life. And isn't that what songs are generally supposed to be about? I write usually ska, so a lot of the songs end up about crazy **** that's happened at parties I've gone to or how funny cool people are when they're acting cool and how funny and brainless it is to see them make fun of uncool people (haha) or other random crap like that, etc. But try that out. If you're writing guitar stuff, forget everything I've just said.