[receiver]は/が + [giver]に/から + [object]を + もらう
The verb もらう means "to receive" or "to get". It describes the action of receiving something from the receiver's perspective. The receiver is the subject of the sentence, and the giver is marked with the particle に or から.
The person who receives is the topic/subject (marked with は or が), and the person who gives is marked with に or から.
Both に and から can mark the giver, but から emphasizes the source and is more common when receiving from organizations or institutions.
もらう is used when the speaker (or someone close to the speaker) is the one receiving. If someone gives something to the speaker, you can use either もらう or くれる – but the perspective differs:
Compare: 田中さんがお菓子をくれた。 (Tanaka gave me sweets.)
The polite form is もらいます. The past polite form is もらいました.
Note: When receiving from someone of significantly higher status, the humble form いただく is used instead of もらう to show greater respect.