how do I clear all data from this computer....I-mac

See instructions in What to do before you sell, give away, or trade in your Mac - Apple Support

Hi, misty9Girl… i can help you on that… i’m presuming you want to give away/sell/dispose of your iMac… first off, you should sign into iTunes and the App store and de-authorize your accounts there… and sign out of iCloud… then make a backup if you want to keep any data/files/etc from your iMac… time machine is good for that… i presume you have your passwords/logins recorded… then run Disk Utility, click on your main hard drive and then click on “Erase”. Go for “secure erase” and that should be it

john b