What should I major in if I want to do one of these jobs in the future? Please Help?

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Majors in college actually don't confine you to certain job fields, but it is nice if they do have some relevance. For marketing/advertising, I would say to study anything with media, communications, marketing, or business. For law, you can study the same things too in addition to either studying history, philosophy, social sciences, other humanities..
And for the secret service, it really doesn't matter what you study, but psychology, social sciences, or forensics are relevant majors.

Majors that will surely apply to all of these fields are:
Communications, Psychology, Social Sciences(Sociology, Anthropology).

I hope I've helped you!