Given son-in-laws old imac, it is still registered under his iCloud and need to switch it to mine.
You can sign out of his iCloud and then in with yours (or create your free iCloud account if you don't have one). This can all be done in System Preferences.


Open the  (Apple) menu in the top left corn
Open System Preferences
Click on iCloud
If your son-in-law's account is still signed in, click the "Sign Out" button in the bottom left corner


At this point, the iMac should be signed out of his iCloud account. On this same screen, you can sign in with your iCloud account and turn on/off the various services you would like to use.

Like a commenter mentioned above, another option is to start fresh by erasing the hard drive. If that is the route you'd like to take, this Apple reference can help walk you through it:
https://support.apple.com/en-us/HT204904