To view how much available disk space there is on your computer, the easiest way is open a new Finder window by clicking on the blue smiling Mac face in the Dock.
This will open a new window. Click on the Go menu and select "Home".
Click on the View menu at the top of the screen and select "Show Status Bar". This may already be showing, so it will say "Hide Status Bar" otherwise, which you do not want to do.
At the bottom of that Finder window it will say how much space is available.
99.48GB available in this example
Click on the View menu at the top of the screen, and select "as List". Then from the View menu select "Show View Options" and check "Calculate All Sizes" and "Show Library Folder". The computer will then calculate the size of each folder. The Library folder is a hidden folder where preference files are stored.
If your Library folder is taking up the most space. There are specific things you can check the size of and remove
1. Library -> Application Support -> MobileSync, this is a backup of your iPhone if you have synchronized it with your computer.
2. Library -> Messages, this is all of your iMessages database if you have your iCloud and messages set up on your computer.
3. Library -> Caches, this is caches created by applications which will get recreated later but can grow in size and not remove itself.
4. Library -> Mail, this is the storage for the Apple Mail program. If you do not use Apple's built in email program then you can remove this.
If it is your Music folder taking up the most space, have you downloaded movies from iTunes? If so you are able to delete them off your computer as they always in your account to download later. They do not have to stay on your computer forever.
The issues of space on your computer are ultimately your responsibility.