‘Used to’ is used for things that happened, or were true, in the past (and now don’t happen or aren’t true)
‘Didn’t use to’ are used for things that didn’t happen, or weren’t true, in the past (but now do happen, or are true).

(note: ‘used’ changes to ‘didn’t use’ because ‘didn’t is the past tense)

Examples of ‘used to’

I used to play football a lot when I was a kid, but I haven’t played for a long time.
He used to like her a lot, but not so much now.
They used to be married. I think they got divorced three years ago.
This city used to be a quiet, peaceful little village. Now it is a hige sprwaling city.

Examples of ‘didn’t use to’

I didn’t use to like chemistry very much, but since the teacher changed I quite like it.
She didn’t use to be so fat.  

Exercises

How have these things changed:
(i) your life?
(ii) the place in which you live?
(iii) your likes and dislikes?