Used to is used to describe an action that was common or ongoing previously but no longer occurring.
Ex. This building used to be a hospital but has been converted into a school.

As a general rule, since the action described is in the past, it is correct to use the past-tense “used to”.
Ex. I used to like dogs, but I got bit by one last month and now they scare me.

The only circumstance you could correctly say “use to” is in combination with the word “did”, since “did” is already in past-tense, “use to” is the proper way to follow it.
Ex. I did use to like dogs, but I got bit by one last month and now they scare me.
This is an informal way of speaking and is not as commonly used.