Your iMac is 3 years older than mine:
Bring up your About this mac, and click on Storage. This will show you how much of your storage you have used. Then click on Manage. This will produce a seperate window like this:
Go through each of the items in the left column to see what you can reasonably offload. One especially good idea is to set your trash to automatically empty after a given period of time; usually 30 days. You can do this by opening Finder and clicking on Finder in the tool bar, then Preferences, then Advanced:
Also, go through the items in the Manage window to get rid of any unwanted stuff, but to use the iCloud where necessary to save important stuff. You are allowed 5gb of space on there for free.
Finally, be sure not to keep any apps on the screen. They can often be still running in the background when you are least aware.
Hope that helps.