Starting a discussion is similar to how all social content is created. In the ACS Network there are multiple ways to do this.
The first way is to use the Create button in the user toolbar. "Discussion" is on top of the list.
If you are already in a forum or group, you may start a discussion by accessing the "Discussion" link directly from your group page.
You should now be presented with a form to start your discussion.
You will be asked where you would like to place this discussion (or other social content). You can search for a group or forum to place it in or you can add it into your own Private Discussions, My Documents, My Videos, etc.
Be sure to tag the content to help it be found. With some forums and groups, you may be given the opportunity to choose the categories this new discussion falls under.
Select "Post" to publish the discussion.
When you create a piece of content, you automatically will be tracking it. This means anytime a user comments on it, you will see a notification in the Activities Stream. You may or may not receive an email, depending on how you have your email preferences set up. SeeSetting Preferences for more information.