Your start up disk is full, and gray screen

I purchased a mac book pro which has OS X mavericks installed on. So I saved everything from my portable HDD in the mac,and shut down.But now every time i turn on the computer, it tells me "your start up disk is full" & enters a gray screen. what should i do?

Note that i used to use a windows computer, so i already made a boot-camp. the computer does open windows OS but not OS x mavericks.
I'm new to the mac.