No one knows for sure except Zimmerman..
What we do know is that Martin walked home from the store after buy iced tea and skittles. Zimmerman, acting as neighborhood watch captain, spotted Martin and decided he was a criminal, even though he was just walking and looking around. He claimed that Martin had his hands in his pants, but he was on the phone with one hand and had his food in the other. Z called 911 from his vehicle to report a suspicious person. (Never said why he was suspicious.) He then got out of his car to follow Martin. The 911 dispatcher (who may or may not have been a cop) said they didn't need him to do that, but he followed Martin anyway. (Legally he could follow him, but it's odd he chose not to follow the advice of a professional.)
Minutes later, Martin was dead. Zimmerman had some minor head wounds (per the doctor who testified). There was a claim of a broken nose, but Z never followed up for confirmation.
Anything else is speculation and what Zimmerman claims happened. A lot of what he claims does not jibe with the evidence at the scene. Zimmerman claims he shot Martin in self defense, that he was afraid of him. He said he was afraid of Martin, but followed him anyway. He said he stretched out Martin's arms after he shot him, but the arms were found under Martin's facedown body. That's undisputed evidence from the trial.
Contrary to what some will claim, a finding of not guilty does not mean he was innocent.
The truth will never be discovered. Zimmerman makes it out that Trayvon attacked him - but it was Zimmerman following the kid through the dark streets that caused all the problems. If Zimmerman had just stopped following as the police dispatcher advised, nothing else would have happened.
Trayvon Martin was heading back to his uncle's house after buying Skittles and a drink from a local convenient store. George Zimmerman was driving around with a gun and spotted Mr. Martin and decided he was "suspicious" (an accusation he has made to 911 some 40+ times about black men & boys - all of which were dismissed by the police) and decided to follow Mr. Martin. Mr. Zimmerman got out of his truck while carrying a gun and confronted Mr. Martin. A struggle ensued (I think most people would take issue with a stranger approaching them while carrying a gun) and Mr. Zimmerman shot & killed Mr. Martin.
Some black kid, dressed like a gang banger was walking thru a SOUTHERN neighborhood. A half witted gun advocate on neighborhood watch determined that he was suspicious. The half witted conservative confronted the young man dressed like a gand banger. The young gang banger look alike was not going to take any abuse from an idiot, and rightfully so. Ad it escalated from there.
Here is a reenactment that uses the evidence of the trial. It's a bit dramatic but it's what i have been saying went on in the first place.
Z killed T. All the rest is just details.