Academic Choices (Advice)

I was hoping I could get some advice regarding some choices I'm planning to make. I recently introduced myself in the Introduce Yourself topic but I'll add some more information in order to make my situation more understandable. All kinds of advice is welcome.

I'm currently living in Mexico where I recently finished what I consider would be my undergraduate studies in the field of psychology.

However I would now like to study neuroscience in the U.S. but I have some questions about that. Is it hard in the US to get any kind of financial aid? Even though I was born in the U.S. and have my citizenship I don't think I qualify as being a resident for a state am I right? Would it be hard\possible to switch from psychology to a neuroscience graduate program, or would it be more recommendable for me to study something more related to chemistry of biology first? Also, is it possible to "transfer" some of the previous subjects/classes I studied if I were to start an undergraduate again? Along with my research I've learned that you can mayor in one thing and minor in another but I'm still confused as to how that works.

Thanks