This pattern describes that you arranged for someone to do something as a favor to you. The focus is on your perspective: you are the receiver of the action. The doer is marked by に (or sometimes から). For example, if you ask a friend to teach you, you say 友達に教えてもらう. It implies you are grateful or at least acknowledge the favor, so it is not used for actions that are obligatory or unwanted.

もらう is a plain form. In polite speech, you use もらいます. To ask for a favor, you can use the て-form with もらえますか or もらえませんか (potential form), which is a common polite request: 手伝ってもらえますか? You can also combine with たい to say you want someone to do something: 直してもらいたい. This is softer than giving a direct command.

Be careful: てもらう is different from てくれる. くれる also means someone does something for you, but it focuses on the giver, and you cannot use it when you ask someone to do something for a third party. もらう is used when you are the recipient (or the recipient is in your in-group).