how to sort out engine light problem?

Hi iv just brought a corsa T reg 1999 and i drive it just feels like its gonna stall when light engine light comes on my car is only a 1.0 and it seems to drink the petrol like its nothing it shudders alot and i just dont know what to do iv put a engine sealer in the engine to see if its coz the heads gone but no still doing it some time it looks like i got water in engine but it goes after while and also oily look in the water so please help really need it