Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
Start up disk means your internal storage or hard drive.
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 Omni Disksweeper 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. Remember, to free start up disk, you must delete some old files. If you get an external hard drive, then we can move the files to that drive and free up space on internal storage.
If you have any difficulty on following the steps above, please let me know and I will walk you through it.