during the scene where sonny is being shot at why does he get out of the car? ive never understood what he was trying to do. Why didn't he try and drive away?

1) Cinematographic reasons: It made an impressive scene even though it's highly unlikely that someone is able to get out of the car with even one machine gun bullet inside his chest.
2) Even if he hadn't been hit he couldn't drive away. His car was stuck between the two cars of Barzini's men.
3) Fight-or-flight response. Sonny wasn't the guy who would sit still. Instinctively he tried to run away just like his father who was more lucky.

Yep.

It was FFC's homage to Arthur Penn, the director of "Bonnie and Clyde," whose famous "dance of death" scene at the end of the film became an icon of violent cinema. The Thompson submachine guns use by Sonny's assassins fired .45 ACP rounds. This ammo is designed for maximum knockdown power. Even one hit would have completely disabled Sonny.

Very true TB. But that's another departure from the novel wherein Sonny is murdered with pistols. Thus, his bursting open of the passenger side door is more believable.

Exactly TB....and in a documentary I saw the prop people were telling James Caan that he would have more explosive caps on him than Warren Beatty had. Apparently it scared the hell out of Caan.

Yes, he's quoted so in "The Annotated Godfather". He played the cool guy but he was really scared. They say that due to the special effects it was the most expensive single scene in the entire movie.

He wasn't in the best condition to think strategy clearly.