The difference is really very subtle. Especially in spoken English, a speaker may use the simple past tense instead of the present perfect, depending perhaps on what they're most interested in conveying.
I opened the doors.
Here, you are relating what you did. It's almost as if you were telling a story. You may have opened the doors recently or a long time ago. They may still be open, or they may be closed.
I've opened the doors.
Here, you've opened the doors recently. The doors are still open as far as you know. It could be that this was something you were asked to do or something you had been planning to do, and you're informing someone that you've done it.
I bought a car.
You're informing someone of what you did, or relating a story. You may have bought the car an hour ago or ten years ago; it's not clear without further information.
I've bought a car.
Here, the point is that you now have a car. You didn't have a car before, and now you have one. Moreover, you've bought it recently; the last time you spoke to this person, you did not have a car. (If you said "I've bought a new car", this would change the meaning completely; it would mean you had a car, but you've recently bought a new one, either in addition to or as a replacement for your old car.)
Frankly, I think that the only way you can learn to distinguish between these two tenses is by reading a great deal. Read as many novels as possible, preferably well-written contemporary novels with a lot of dialogue. Over time, you will develop a feel for which one to use when.
Edited to add: The above is not a set of rules. Rather, it is an attempt to describe what a native speaker thinks when they hear the quoted sentences on their own, without any context. There is a lot of overlap between the two tenses if no context is given. In a given context, however, one of them may be wrong. For example, if you say, "Before, I used to walk to work, but now I've bought a car," you have to use the past perfect. If you ask someone, "Where did you go?" when talking about the previous weekend, you have to use the past simple. Since there is no hard and fast rule that can explain all this, I reiterate my suggestion that the best, and perhaps the only way, for a non-native speaker to learn to use these tenses correctly is to read hundreds if not thousands of books.