There are many ways to do what you have requested much of it depends on your level of expertise in the development of dynamic web pages.
You could setup up a log in system to access the main site, based on a user submitting a email address, with a login and password, and then have the email address verified, once that occurs, the login and password become active a new selection appears on the landing page giving them access to the main site.
But I have question the logic behind this type of system. The likely hood that people will give you a valid email address and a logging into a system that they have no idea what the content is highly suspect. I have seen a few sites like this in my travels on the web and I immediately leave those sites.
You would be better served to have them enter your site and browse through what is available wet their appetite with your content, get them interested in the subject and then have them given the option to get additional information with a popup that requests an email address and a login to a protected section of the site.
Again there are many ways that this can be accomplished based on your level of knowledge in site development.
Let me know your background and I can get more particular for you.