I am a victim of ransomware and malware.

A few weeks ago I went to mediafire to download something, not a virus, but something. When I clicked download it opened a little chrome window that said I needed to authenticate, open a certain command app (not cmd) and it copied something on my computer and told me to paste it into a command prompt (it was a random script I can give you) and it injected something. I didn’t mean to do it that way, I did it all on instinct. Anyway. Around August 9th I got an email that showed me all the passwords I’ve ever used and had at the time, and it showed me a picture of my desktop while I was using it, and there was another picture where they were asking for a ransom of 900 dollars, and I had to pay it within 48 hours or they would release all the passwords I’ve ever had. But I didn’t see this email until TODAY, and I saw it in a spam folder, and it said it was August 30th. But the day of the attacks (on August 9th) when I started receiving emails with credentials on Microsoft, other apps, games, saying that people were trying to log into my accounts from different regions around the world. Well, I really need some advice on how to protect myself from potential threats and attacks like this. Maybe an Anti Malware application that is free and protects my passwords, emails, web searches, stuff I download and such. But an efficient one.