Hi Rosaria,
If a visitor attempts to visit a part of your site that is protected by s2Member, they should automatically get redirected to the Membership Options Page.
However, you said you’d like to restrict part of your posts but you also don’t want people to see your site. If visitors are forced to register when visiting your site, then who do you want to see your partially restricted posts?
If you don’t want to show the blog posts at all on the home page, then I suggest changing the home page to a static page (see Creating a Static Front Page). It’s not recommended to use the Membership Options Page as your front page as it causes internal conflicts with s2Member. If you want to show your Membership Options content on the front page, you can copy it over to a separate page.
Finally, if you’d like a more advanced solution, you can use the s2Member Advanced/PHP Conditionals and Advanced/PHP Query Conditionals (see WP Admin -> s2Member -> API/Scripting) within a Custom Page Template (see Creating Your Own Page Templates) to customize the output of the blog post listing. Then you can set your front page to a Static Page that uses your Custom Page Template to display blog posts as you see fit.