What is the difference between は and が???

When you translate“I am Smith”into Japanese, Some of you will feel confused about the difference between わたしはスミスですand わたしがスミスです。. Actually, we Japanese can easily identify this difference without notice, however it is difficult to give an answer.Today, I would like to give an explanation for this question.
Frankly speaking, this difference is which noun is more important.

I am smith.

Noun1 Verb Noun2. Right??

And as well ,わたしはスミスです、わたしがスミスです are Noun 1 は（が）  Noun2 Verb.
So , what I want to mention is that structures are same between わたしはスミスです、わたしがスミスです. However , for us, the focus are different.
For example, one worker visit his customer’s house and try to introduce himself, “I am Smith”.
In the view of this customer, she must be sure that he is there, however she is not sure Who is he,
That is why he introduce himself to her. It means, in this case, we should focus on Noun2 and we should use わたしはスミスです.
Let’s consider another example. Before he visit, he call her and inform her that he will go to her house with his colleagues. After he get to her house, she must be sure that there is Smith, however, she is not sure who is Smith. It means that we should focus on Noun1 , and use わたしがスミスです.
Let’s summarize. If you want to focus on Noun2, like the first example, you should use,
わたしはスミスです（は）.
If you want to focus on Noun 1, like the second question, you should use ,わたしがスミスです。（が）
This is the difference how to use は and が.

That’s all, Thank you for reading.
I am really looking forward to your request , question and review.