Dear Kuba,
to create submenu items, please go to “Appearance -> Menus” admin section and move the needed menu item slightly to the right with the mouse until it “sticks” in such position and save the menu – please see this screenshot for illustration.
Regarding your second question: creating a blog post that would not be visible on the blog page but will appear in the menu is not possible in WordPress; it seems like what you need instead is a page, not a post – in this way it will only be accessible via the menu if you put it there. You can create a page in the “Pages -> Add New” section of the admin interface.
Sincerely,
Andrew / Satori Studio