You may get a message that your your Mac OS X startup disk is full (or your startup disk is almost full). Don’t panic. The good news is you can easily free up a lot of space. First, find out how much hard drive space you have. OS X should have at least 2 GB available space to function properly.
Please this article and follow the steps. Also see this article. Also note that If you have a lot f media files (movies, musics, pictures etc) stored on your hard drive, they may take up a lot of space. You may want to consider them to another hard drive. If you want to do that, see these articles: