how to free up memory on iMac?

Give this a try:

• On the menu bar, click on the apple icon
• Click on SystemPreferences, Users and groups
• Unlock the padlock
• Click on login items
• Select a login item and click on the - sign

Do this for all the applications that log in automatically

Reduce the amount of startup items in System Preferences - Users & Groups - select the user - click the Login Items tab. Then select items that do not need to loaded when you startup the iMac. To remove them simply highlight the item and click the " - " symbol below. If you have a 27" iMac and it still has the base 8GB of RAM, purchase an additional 16 GB of RAM (2 X 8 GB DIMMS) from either Crucial.com or Macsales.com and install the new RAM. This takes about 5 minutes and requires no tools on 2012 and later 27" iMacs.

Reboot your Mac.

This message comes up when I first start my iMac.