What should I major in if I’m interested in researching immortality for humans?

Immorality?

Philosophy.

I suspect either genetics or cell biology, if not both, would be the most fruitful area of research. Most of the effects of aging are, in essence, when our cells stop being able to make enough good copies of themselves. So if there's a "magic bullet" that would halt or reverse aging, it will probably be in one of those areas.

Not a good idea. There was one biologist who found that he could dramatically increase longevity in porpoises by feeding the seagull meat. One day he was bringing a batch of the birds back to his lab when he saw a lion lying across the entrance. The lion looked at him and then lay its head down. The biologist figured that it was safe and stepped over the lion. The lion did not bother him but he was arrested for violation of the Mann Act, transporting gulls over a staid lion for immortal porpoises.

Probably would want to study the fundamentals of all sciences to get a very rich background in science. Examples being Biology, Chemistry, Physics, Mathematics, etc...

I think, for research like this, you would probably want to broaden your research to all animals instead of just humans. There are animals that are animals that are virtually immortal.