Legally, Under Florida's SYG law, it does not matter who started the fight. But, "who started it" is a somewhat interesting question. There will never be an absolutely certain answer to the question, however, when looking at Zimm vs Martin my money is on Martin starting it just like Zimmerman said he did, by sucker punching Zimm and knocking him to the pavement. It is obvious that Zimmerman never landed a single punch. If he had, there would have been bruising on Martin's unscathed body.
It is just simple self defense unless they prove Zimmerman is lying. If they can't prove that Zimmerman started the violence then he isn't guilty of anything. Following people isn't illegal in this situation. Unless the prosecution can prove that Martin didn't hold him down and beat his ass then there is no case at all.