Ask HN: How can I make legally valid Terms and Conditions and a Privacy Policy

I have a web app that I've created and published to the internet.

I've made a short Terms & Conditions document, along with a Privacy Policy. New users are obliged to agree to both when creating an account. I question whether these documents are legally valid, since I wrote them myself without following any specific structure.

Is there a way to make legally valid versions of these documents? What is the best way to approach this in order to ensure I am not legally liable for anything innapropriate that users publish to my site?