There should be a rubber gasket between the manifold and the head. If that isn't there, that's the problem. If it is there and still leaking, you most likely tightened the screws too tight and/or the gasket is broken. When tightening the screws you should stop as soon as the screw peeks up over the bolt. Only like a 1/64th of the screw should poke up over the bolt. I good way to know you have over tightened is the manifold will be bending. It should lay nice and flat.