I'm a sophmore in highschool and I'm thinking about a fighter pilot career, but I really don't know that much about the requirements. And I don't know if it's even possible for me now because of several things.

My grades were good (not great) last year, but this year they're horrible becuase I was in a rather negative state of mind for most of the year and I didn't really care.

I'm not good at stuff like algebra, but I'm fine with basic math.
I heard you have to have a real high GPA and take a lot of math to even have a chance at being accepted into an academy.

So my first question is: Is it worth my time to raise my grades and what are the chances of me getting accepted if I do?

And exactly what are the requirements for being a fighter pilot? I hear they're different depending on what military branch you're in, like Navy, USAF, the Marines, etc.

My best advice is to raise your stats as much as you can.  Admissions will take large improvements into consideration.  You should try to make it to at least trig/pre-calc in math.  Calc 1 is recommended, but not absolutely required.  If you do not make it into an academy, there are other options such as ROTC.  You can join ROTC in college (and get scholarships). 

Medical requirements for pilot qualification vary a little between the services.