https://medium.com/@patalcala/dont-pay-the-ransom-ransomware-awareness-part-1-identification-230274406918
A guide on how to prevent and fight ransomware for non-technical people
They will demand payment in cryptocurrency, usually bitcoin, which is untraceable. There's no guarantee that they will really keep their word; since it's completely anonymous, there's nothing you can do. If you get infected, you just have to accept that your files are gone. The best thing you can do is backup while you still can.
Make sure some of your backups are kept offline (so that they won't be encrypted along with everything else) and also test that they work regularly! Otherwise they might not do what you think they do.