Your unit can have multiple forums and topics, but you must create a forum before you can create a topic since all topics belong to forums.
If the updated group discussions interface has been turned on by your administrator:

1. On the navbar, click Discussions.
2. On the Discussions List page, from the New button, click New Topic.
3. From the drop-down list, select the forum you want to put your topic in. If you want to create a new forum for your topic, click New Forum.
4. To grant everyone access to the topic and restrict learners to only see threads from their own group, under Topic Type, select Group or section topic. Note: You cannot edit the topic type once you save the topic.
5. Enter your New Topic Details.
6. On the Restrictions tab, select Availability options for your topic.
7. Select Locking Options for your topic. Locking a topic prevents users from posting to it until it is unlocked; they can still read post made prior to it being locked.
8. Click Save and Close.

If you are using the default group discussions interface:

1. On the navbar, click Discussions.
2. On the Discussions List page, from the New button, click New Topic.
3. From the drop-down list, select the forum you want to place your topic in. If you want to create a new forum for your topic, click New Forum.
4. Enter your New Topic Details.
5. On the Restrictions tab, select Availability options for your topic.
6. Select Locking Options for your topic. Locking a topic prevents users from posting to it until it is unlocked; they can still read post made prior to it being locked.
7. Click Save and Close.

Watch how to create a discussion topic

https://www.youtube.com/watch?v=lOQdYOVPuCE&feature=emb_title