With posts, to the best of my knowledge, this cannot be done. However, it can be done very easily with pages:
- Create the first page (page-title)
- Create the second page (subpage1)
On the right-hand side, you should see a box called "Page Attributes". There is a section called "Parent". Choose "post-title" from there, and voilà. The page structure and everything should follow. Repeat for as many sub-pages as you wish.
By the way, the technical term for this in WordPress is called "hierarchy", and you can find a lot more information by searching for that term. Here is a good place to start.