My fish isn't looking to good. what should i do?

My friend gave me a fish yesterday i'm not sure what kind of fish it is. maybe a goldfish with black spots but it is fat with 3 bits for its tail. Anyway it is lying near the top of the water and has been for about half an hour. before that it seemed happy swimming around. I don't know whats wrong with it but its tail is drooping and only moves a tiny bit every now and again. Can anyone help me and my poor little fish?