Why is my car idling when parked or at a complete stop? What needs to be done ti fix this?

When I accelerate it stops, and I also bought something to cleanse the fuel? I'm not sure what it was called, but I put it in the fuel tank and where the oil goes. I know nothing about cars and any help would be greatly appreciated!

It shakes really bad when I am parked, or for example at a red light. When I accelerate, it seems to stop. It almost feels like it is about to die or as if I am running out of gas. I relaced the battery today and it is still doing the same thing. I opened the hood and the engine is shaking BADLY! I know something is wrong I just have no idea what it is.

No the check engine light is not on. My friend did take a look and said something about a gasket that was leaking! I don't know if this helps, but when I put it in neutral it runs a lot better but the problem is still clearly there.