There are lots of great resources to learn to factcheck. Here's a 30 minute crash course to really bring you up to speed.
Note: this is a bit of date and says some things about News Detective that don't apply any more (Don't worry, all the actual content about fact-checking is fine.)
If you don't have time for that, here's a 3 minute video to get you started.
News Detective is for uncovering misinformation. This is not a general interest question-answer site.
Posting
The title is the "main claim" that you're trying to factcheck.
Example:
Factcheck This: Birds don't exist
If possible, LINK TO to the place you saw the claim.
Answering
LINK TO YOUR EVIDENCE or otherwise explain the source ("I called this person, I found it in this book, etc.")
But don't just drop a link. Give an explanation, copy and paste the relevant information, etc.
News Detective is not responsible for anything anyone posts on the platform.