I had an email stating that I was being held to ransom

If you received an email or phone call or text message claiming to be from Apple Computer with charges or claims you do not recognize or with requests to provide or change personal information DO NOT click on any links or respond to it.

Avoid phishing emails, fake ‘virus‘ alerts, phony support calls, and other scams

https://support.apple.com/en-us/ht204759 

Apple's security updates are the single best software defense against malware, however it is only a good as what the developers have secured it for. Keep in mind threats that your system has been compromised don't need to be based on any actual technology. If you get someone to believe they have been targeted and have you go to a phony page to capture your account information then there is little you can do about it with software. Common sense is necessary.

It is a hoax. Ignore it as you would with any other piece of spam email.

Do you believe everything sent to you in an email?

Phony "tech support" / "ransomware" popup… - Apple Community

I was wondering how secure my iMac would be?

"Security" is a very broad concept. Start by reading Effective defenses against malware and other threats.

You’re welcome.

The only thing that stood out with this was that they had my email address totally correct.

How else would you have received that email?