Need help/confused about marketing

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I want to pursue my mba specialization in marketing field but I was told by my brother to first know about what exactly do I want to do in marketing field? Whether I want to be a product manager, digital marketer, or any other niche of marketing? But I have no clue of what to opt for as of now. I was actually thinking that once I get into marketing, I can choose on the basis of interest I develop thereby after studying various subjects but he told me to not go for that route. What should I do now? Please help me by letting me know about various niche of marketing roles & what exactly a person is required to do in that role? I would really appreciate your help.