How does ransomware get on your computer?

99 times out of 100, ransomware gets on your computer when you deliberately choose to put it there.

You don’t know that’s what you’re doing. You’re tricked into it. But you choose to put it there. You voluntarily, of your own free will, allow it to run on your computer.

Some of the ways this happens:

• You receive an email that says it’s from the IRS. You are being investigated for tax fraud. If you do not pay the attached invoice, an arrest warrant will be issued for you. Oh no! You get a jolt of adrenaline. Fear inhibits the prefrontal cortex. You stupidly click the “invoice,” which has a name designed to fool you like “tax-invoice-11278484-official-government-document.pdf.cab”, and boom, you’re infected.
• You receive an email that says “I paid you the money but I never received my product. I’ve attached my bank statement to prove I paid you.” Same thing. You click on the “bank statement.” It’s not a bank statement, of course, it’s ransomware.
• You’re browsing the web. A popup appears: “Your copy of Adobe Flash is out of date. Click here to update. If you don’t update you will not be able to watch movies.” You click on the updated. Boom! You’ve just infected yourself. (Hint: there are never ever ever any real Adobe Flash player updates ever. Adobe discontinued Flash years ago. I do not understand why this trick still works.)
• You’re browsing the web. A window appears: “WARNING!!!! CRITICAL SYSTEM ERROR!!!! Your computer is infected with 8 COMPUTER VIRUSES! Your files will be destroyed! Click here to download Norton® Mcaffee® Antivirus by Avast® to fix this problem!” It’s a lie, of course. You aren’t infected. But you’re scared, and scared people are stupid, and so you click to install the “antivirus,” which is of course ransomware. Congratulations! You have just been tricked into voluntarily infecting yourself by your own choice of your own free will.
• You want to run Microsoft Office but you don’t want to pay for it. So you Google how to get it for free and find yourself on a site that advertises a cracked version of Office (or a game or Photoshop or whatever). The crack includes a bonus surprise: ransomware!

Basically, almost all ransomware infections are some sort of variation on this idea. Someone lies to you to trick you into infecting yourself.