Where would I insert an exclamation mark and a comma in the below sentence?
"Look at that boy" he exclaimed.
According to Oxford, you can add the exclamation mark if you want to express that the "speech" is shouted or spoken loudly. I believe that it can emerge a drammatic effect in your writing. But if you consider it as a plain command, just use a period.
Regarding the comma, it should be inserted in the closing quotation marks(I have no references for this, but I read lots of novels, and they do so).
But don't use two punctuations in a row.
"Look at the boy!," she exclaimed" is wrong.