Invitations to sign up for these three sites will be sent by email to members of the Church over the next few months.
Update June 5, 2014: This post previously recommended that you change your name and email settings in FamilySearch.org in preparation for receiving your invitation. I have confirmed with FamilySearch that these steps are not necessary.
When you receive your email invitation, it will explain how to register for your personal account. The invitation e-mail will only work for the member who receives it.
For more information see LDS Member Access to Partner Family History Websites.