Report buttons are in several locations in game such in chat, the lobby browser, in-game, and the post-game. Each report includes check boxes to select the reason and an area to include any additional information. These reports are reviewed by a moderator before any action is taken.
Actions are taken based on the Microsoft, and Xbox policies. If you have any questions about what is considered acceptable (and unacceptable) behavior reference these policies.
If the report needs more information than can be provided in game, then send an email to aoesupport@microsoft.com. Please include your Xbox gamertag or Steam ID along with the date and time of the match and additional details about the issue.