Ask HN: What does a newbie need to know in order to avoid AWS cost disasters?

I am about to use some AWS services for the first time and I am a bit worried about the disasters that happen because of the pay-for-what-you-use business modal, a simple loop can cause you a 5 figure bill.

So I am asking about the necessary things I should know as a beginner AWS user to avoid such problems.

Also, any other info/advice to reduce costs is welcome.

The services I will be using are S3, Elemental MediaConvert, and CloudFront.

For video streaming.