Hi Tiffany, to delete a topic, go to that topic and click the "Moderate topic" button in the right hand column. This will take you to the Moderate area where you will see the topic with a grey cog icon in the upper right.
Click the cog icon and it will show a list of options. Select "Remove" and then choose how you would like to remove it. You can choose "Delete" and it will permanently delete the topic from your community. If you select any of the other options such as "Spam" or "Duplicate" -- the topic will be removed from your community, but not permanently deleted. It will be moved to the "Removed" tab inside the Moderate area, which allows you to view it later. I hope this helps and please let me know if you need anything to be clarified.