I have no idea what to write about....I keep having thoughts of writing about love/heart break but all I can think of is so....mainstream and I'm sure has been used before...I want to write something that doesn't just say "I'm sad because she left" or something like that...

Any advice would be great.

My advice would be to write something mainstream and that has been used before. This will help you getting used to writing songs and then you can improve. It's like playing guitar, you had to learn how to do the basics in order to understand the bigger picture.
it has to mean something to you. you can't expect other people to do the hardest part for you.
go to a store and shoplift, and after you get out the doors while you still have that adrenaline going, write down the first twenty lines that come to you, quick - and don't get caught!