Put Blogposts into Category Folder

Hey,

right now I have my URL structure like this:

mainpage.com/categoryname

which shows my blogposts from that category. But when I click on one post I get redirected to

mainpage.com/blogpostname

but I want to put topically similar posts into the same category like this:

mainpage.com/categoryname/blogpostname

or potentially

mainpage.com/categoryname/subcategoryname/blogpostname.

This must be simple.. would be happy if someone could help me out!

Thx,
Enrico