I want to confirm if I'm parsing this sentence correctly. It's probably the difficulty of not having commas to break it up.
Loosely, I'm translating this as
"If you want someone to like you, you should first like them."
好きになってもらいたいなら - If wanting to receive someone's liking
まず人を好きになる - First like people.
Also it's not clear if 好き in this context is more of a generally being liked by people vs a liked by a single person?
"If you want people to like you, like others first"