Use Save as Draft to save progress on a post that you are not yet ready to publish to other site participants.
Select the Conversations tool from the Tool Menu of your site.
See How do I post a topic in Conversations? for the complete steps to posting a new topic.
Topics that you have saved as drafts will appear in the DRAFT area of the topic list. Draft topics will also display the message "This topic is a draft" above the post when you select it for viewing. Draft items are only visible to the author. Click on a draft topic in the topic list to continue editing and/or publish it.