Hello. I am new to this site, but I saw your question and thought I could help. Using "used to" is an easier way to discuss the past tense in English, without learning the past tense of each verb. Example: "I went to the movies" versus "I used to go to the movies". Also, "I used to have a dog" versus "I had a dog". A native English speaker will understand you, but you should learn the past tense form eventually. I hope this helps.

'I used to have a car' implies that you once had a car, but no longer have a car. 'I had a car' is more opened ended. "I had a car (when I was 20 years old/a year ago that broke down)" They can mean the same thing, but 'I used to have a car' has more implication to it. hope this helps! :)

Both are correct and fully interchangeable. I should emphasise that if you're trying to say you owned a car over a prolonged period of time the 'used to' form is more common while as 'had' simply indicates you had a car at one point, maybe for a minute or 10 :-)