how can i remove items from my icloud storage?

Hi, I was advised by Squaretrade (my iphone insurance) that my problem of extremely low volume could be fixed by doing a factory reset. In order to do so, I would need to back up my phone first, which I can't because my icloud storage is full. I also can't update since I have no storage. My question is this....how can I remove things from my icloud storage? where can I view what's being stored?