Here is an example that can illustrate the difference between the Past Simple and Present Perfect.
I have just called him. (no specific time is mentioned; the keyword "just" is used; the action has taken place right before the statement is uttered)
I called him at 1 pm. (it is specifically mentioned when the action took place).
Getting accustomed to using any of the tenses of the English language takes some time. When it comes to the Present Perfect, there is always an aura of something "warm" in the air. In the case of the Past Simple, everything is "cold". Of course, you might disagree with me and say that something that happened just a minute ago is still "warm". And you're right about that. However, if the time (e.g., 1 pm) is specifically MENTIONED, you'll have to use the Past Simple.