Before you start building your site, it's a good idea to identify exactly what you want it to do. For example:
Will all content be restricted to members or will some be available to all?
Will you charge for membership?
Will you have different membership levels with different content?
Will your site include advertising (maybe with no advertising for members)?
Will you offer free membership?
What will you do to communicate with your members? Will you send them notifications of new content?
Will you use the data you have about your members to sell products? Make sure you have permission from your members if you do this.
Will you let your members talk to each other? You might want to think about installing BuddyPress if you want an interactive membership site; if it is the case, refer to Chapter 2, Creating a Social Media Site.
Once you've identified how you want your membership site to work, you'll be able to find a plugin with the features that you need....