Both “Det er min bil” and “Det er bilen min” are correct. Please notice that in the first sentence “bil” is in it's undetermined form, and in the second sentence it is in it’s determined form. The difference is that in the first sentence you are talking about who owns the car, and in the second sentence you are talking about the car. If it makes it easier to remember, you can look at which word comes first and think that word is more important. So when saying “Det er min bil”, you tell the listener that the car is yours, and that it doesn't belong to anyone else. When saying “Det er bilen min”, you are simply explaining that what you are talking about or what they see is your car, and not anything else. :)