Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
Start up disk is your hard disk and you need to have atleast 10GB free space for proper working of OS X. To free up space, you have to delete unwanted files, applications, music, movies etc and then empty the trash.
First let's check what is taking up all the space. For that we're going to use an application named OmniDiskSweeper.
Download OmniDisksweeper from here --> https://www.omnigroup.com/more
Use the trial or free version. That application will show the large files that are using the space of your hard drive. So you have to find and remove those unwanted big files that you are not using and empty the trash.
DO NOT delete any system files from System, Library, Private folders etc. If you delete any system files, your Mac will not boot or load. Only delete files which are in your home folder.
Scan with OmniDiskSweeper and it will give you an idea of what is taking up space. It will show large files and its location. Note it down.
Then in Finder, click on GO menu at the top and select Home. This will take you to the home folder. Open each folder and delete unwanted files from there and empty the Trash.
DO NOT trash iPhoto Library, Photos Library or iTunes Library.
Make sure you empty the trash after deleting unwanted files and stuffs
Let me know how it goes.
Note : This is not my final answer. I strive to provide Excellent service or 5 star service. If you feel you've received anything less or if you are not satisfied or if my solutions didn't work, please don't rate me bad or poor or ok. Instead, please let me know and I will try my best to help you until you are fully satisfied or the issue is resolved.