Should there be a comma when I write:
"In this talk, I will briefly explain how to safely build a computer from scratch."
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The choice is entirely up to you. Usually, a comma is placed after an introductory adverbial (here: in this talk) if that adverbial is long. By placing a comma you then improve the readability of your sentence. In your case, the comma is not necessary, but if you do place it you're telling the reader to pause briefly when reaching the comma. Without the comma there will not be as much of a pause between talk and I.
So it all depends on what you want the flow of your sentence to be. Do you want the pause there or not? It might also emphasize just that bit more that it is in this talk you will be doing the explaining and not in any other talk.
It is permissible to omit the comma after a brief introductory element if the omission does not result in confusion or hesitancy in reading.
If there is ever any doubt, use the comma, as it is always correct.
To ensure the usage of comma, this is a good site for you.