In order to report someone on Discord, there are a few pieces of information you'll need. You will need their user ID, a link to the message you are reporting, and the server ID. This information can only be access by putting your Discord into Developer Mode (Settings > Appearance > Developer Mode). From there, you will be able to right-click the user, click the three dots on a message, and right-click the server name, to get all of that information.
Once you have all of this information, go to Discord's Submit a Request page and fill in the required details.