@allchange
This must be a terrible time for you, no question. I think all of us here in the Forums are striving to mitigate the damage that could happen to you.
Stay with us and we shall help.
1. Create a Backup (BU) of all your data (and settings if possible). You haven't said whether you have Time Machine which is part of the Apple system, or some other BU process. But a BU is essential.
2. Erasing the Mac's internal drive and reinstalling the Operating System (OS). To assist you in this, the process is the same as if you were selling the Mac. Read this Apple Link below, carefully and follow the instructions. Once you have finished, you will have a "Welcome" window the same as if you had just bought the Mac. Then follow the instructions about downloading your data and settings from your BU - Time Machine is the easiest.
Reset your Mac to factory settings to prepare it for a new owner.
support.apple.com
That part is "relatively" easy. The tiresome part is the changing of all your Passwords (PW) that exist within Keychain. Some may be of little interest or danger to you; but all PWs which are linked to personal details, banks, subscriptions, and - of course - your Apple ID will need changing.
My heart bleeds for you because that part is the hardest of all.
If we can help you in any way along the line, post back and we'll do all we can.
Ian