why is my truck engine smoking? on 1998 GMC Sonoma

i added coolant in the radiator and the reservoir. My oil is low. it starts ok but when i start it starts smoking. i am wondering what do i have to do.

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2 answers
Somke or steam?..... Find out where the coolant went!..... May want to get a combustion leak and pressure test's on the cooling system at a repair shop..... Possible head gasket issue!
have you ever overheated eng and how many miles on veh?