〜てもらう
have someone do, get someone to do
Receiver は + doer に + Verb て-form + もらう
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てもらう is receiving an action: 友達に宿題を手伝ってもらいました, "I got a friend to help with my homework". You are the subject; the person who did it takes に.
It's the other side of てくれる. 友達が手伝ってくれた and 友達に手伝ってもらった describe the same favour. てもらう puts the focus on you, often implying you asked: 医者に見てもらった, "I had a doctor look at it".
It's also how Japanese says "get something done by someone": 髪を切ってもらった, "I got my hair cut".
The potential version, てもらえますか / てもらえませんか, is a polite request, the next point.
Examples
友達に宿題を手伝ってもらいました。
I got a friend to help me with my homework.
一度、医者に見てもらったほうがいいですよ。
You should get a doctor to look at it.
兄に車を貸してもらいました。
My older brother lent me his car.
日本人の友達に作文を直してもらいました。
I had a Japanese friend correct my essay.
お店の人に写真を撮ってもらった。
I asked someone from the shop to take a photo of us.