I'm understanding that you're asking on how to make a report in-game.
To do this you can simply type /report in chat.
If by any chance there isn't an admin available, yet you see a rulebreaker. Please capture video or screenshot evidence. Easiest way of getting a screenshot is F12 or the PrintScr button. Afterwards come to the forums here and go to the report a player section, where you can, following a template, report a player for any kind of rulebreaking.