Engine its not running well

All I did to it was I changed the sparks plugs wires coils and the valve cover gasket and after that the engine starts to running poor and the smell of gas come out the exhaust pipe if I put the car in drive and I dont keep my foot on the gas pedal teh engine shoot of by it self and the needle of the rpms goes donw to 200rpms instead of 1000rpms
so could you please help me ro resolve it thank you