Depending on the nature of the bug, there's a few main factors that are interesting:
A description of the bug, what you're doing, what's happening (or not happening) and why that's different than what you think should (not) be happening.
An explanation of the steps of things you do before it happening (QA wants to reproduce an issue to both verify it being a bug and pinpoint what might be causing it)
And further it can help sharing a save file from the moment, which you can find by typing '%LocalAppData%\Larian Studios\Baldur's Gate 3' into the explorer address bar in windows. Saves are under PlayerProfiles, then your profile for that playthrough. (Remember to include both the .lsv and .png, both are required)
Those are like the most rudimentary things that are good to share.
Edit: Lastly it's best to send bug reports to this link
IMO, easier to work with reports there.