Answer:
You
Explanation:
The subject in the sentence would be you since you are the one who will be performing the act.
Meanwhile, the computer is an object which is not performing the act of turning itself off.
Also, turn is not the subject either since turn is an adjective, which is used to describe/modify a noun or pronoun.
Thus, the only thing left on the list would be you, so it's only logical if that's the subject. Yes, it is not a part of the sentence, but we can infer that the act will be performed by you, thus making it the subject of the sentence.
