Why won't the ios8 download on iPhone 5s through the new MacBook Pro?

Hi! So, my macbook pro is always sending notifications that says that the start up disk is full. I have tried to minimize the contents to the point that sometimes I don't even have the things I need on here because I have been deleting files like crazy to have some open space. This has also prevented me from installing ios8 on my iPhone. Please help.(P.S. all my softwares are up to date)