What major(s) and qualifications do you need to be a trader at a top hedge fund?
Probably at least a bachelors, and preferably a masters, degree from a good university plus your major should be in finance, economics, accounting, and maybe business administration. You will also need to pass certain NASD examinations, never have been involved in any financial fraud, and provide a personal and employment history that they can verify. They may also require an analysis of your credit, investment portfolio, and a few other items.
Do you actually mean a trader or are you asking about a fund manager? A trader executes stock trades. He will need to be fast, efficient and have good arithmetic. Being able to multi task is important as is the ability to stay calm under pressure. These are personality characteristics not qualifications. Often he will be someone who was a star athlete especially in a team sport. A degree is not a requirement. The best are often ADHD. If you meant to ask about a fund manager, these people start as analysts following a sector. Depending on the sector they might have a degree in Chemistry, Physics, Biology or Maths. Generally they will have attended an ivy league university and will have done internships. They will also have good sales skills as they will expected to spend most of their time fundraising.
Assuming you weren't born into Scrooge McDuck type of money, you will need top undergrad finance degree, probably a master's in economics, finances, business, have your trader licenses, and some old college friends working at the hedgefund who were born into Scrooge McDuck like money never hurts either.