Split a blog in 2 parts and require login for 1 part

Hello,

I am relativily new to WordPress and want to build a website with a normal blog (for posting news) and a second blog on a page that can only be accessed by registered users.
And in a perfect world there would only be one login name and password (for all users) and logged in users should still fill in a name when commenting on a post.
Don’t know if this makes sence to you…

I hope the splitting of the blog can be done by category so I can make even more seperate blog in the future.

I hope someone can help me out!