We seem to be in a plague of scammers at present. Every week I come across yet another person who let their guard down and got scammed.
Generally it is one of three things: a pop-up on the computer telling you that your computer has problems that must be addressed immediately, or a phone call on the landline saying the same thing, or an email coercing you to click on a link, this is called 'phishing'.
These people are very good at what they do and what they do is con money out of you.
NO MATTER HOW PLAUSIBLE IT SOUNDS THEY ARE ALL SCAMS!
Don't think you are immune, all it takes is a weak moment, tiredness, a lack of focus and they're in. Most people I have spoken to about this say that they knew something wasn't right but they went ahead anyway until it was too late.
Too late means you have given them your credit card details...
The rather unpleasant league table I keep has $4,000 at the top followed by $1,000 and quite a few $300 and $400 losses. These are local people like you and me who have lost these amounts.
Two things to do that will assist you is to run software that finds Adware on your computer, called Malwarebytes and then install Avast anti-virus software which runs all the time. Both are free and these links take you to the pages that explain how to install them.
Things are changing: although we don't have any real viruses on the Mac there is Adware from dodgy websites and nasty scams via email and the good old telephone.