How can I waterproof my ceiling?

Excuse me, I'm curious about waterproofing my ceiling. I've heard it's important to protect against water damage, but I'm not sure where to start. Could you please elaborate on the steps I need to take to ensure my ceiling is properly waterproofed? Are there any specific materials or products I should use? Additionally, are there any common mistakes I should avoid during the process? Thank you for your help!