How do you fix an antifreeze leak in your car?

Depends where it's leaking, but basically you fix it the same way you fix any coolant leak.

<><><><><>

Generally, its a hose that is leaking. You find the leaking hose, and you replace it, being sure to properly treat the ends with anti-leak compound. If its the radiator, you need to replace that. If its the intake manifold or head, you are getting into big bucks to replace the gasket involved.