Step one is to unplug the machine from the internet. Step two is to use another machine to change all your passwords, starting with the “pivot” passwords - your password manager master password, your email accounts, your AppleID, your mobile provider - followed by financial accounts and then all others. While changing passwords, make sure to “invalidate all sessions” where possible.

Only after you’ve done all this should you move onto Step 3: reformat your computer and install the OS from scratch.