Hi Olga
I personally don't use the term "didn't use to" often. When talking in the past, if I used to do something for example, going to church as a child, I could say the following:
"I used to go to church when I was a child" - this would be taken as something I did often.
"I used to go to church sometimes when I was a child" - adding "sometimes" would let the listener know it was something that I did, but not often.
"I never went to church as a child" - to state something that you did not ever do in the past.
If I were to say "didn't use to" do something, it would be in a situation where I do that action in the present. For example:
I didn't use to smoke, but now I do.
I didn't use to jog, but my doctor said that I should start exercising to stay healthy.
Otherwise, if it is an action I never did in the past, I would use "never"
I never went swimming as a child
I never stole anything in my life.
I never took a French course in school.
Hope this helps.