You can only initiate discussions in a community of which you are a member.

After you log in, you will see your Dashboard.

You can then go to your community by selecting it from the list of the communities you belong to in the "Community membership" section...

Or clicking on "Your communities" tab.

Go to the community you want to post your discussion. 

On the left there is the Community menu:

Click on “Discussions” which will display all discussions. At the top of the list of discussions is the button “Add a discussion” 

Clicking on that, you will get the "Add a discussion" page, where you fill the discussion title,
add tags to link to other discussions and inform the Discussion message, where you’ll explain your discussion and what your expected outcomes are.

Below it, is the "Discussion Status" option. 

An open discussion allows members to post comments. 

The “Access” field allows you to choose the level of visibility of the discussion 

1. Visible to just the community you are into; 

2. "Private" is recommended for drafting purposes; 

3. "Logged in users" means the discussion is visible to all logged members of the entire platform;

4. And "Public", that allows visitors to access the discussion as well.

Click on save and your discussion is created.

Now, to keep a discussion going, you need to motivate people, provide feedback and frequent updates.

If you'd like to reply to a discussion, scroll to the bottom, put in your comments and click on Post.