I think Goku would have to atleast be Super Saiyan 2 to kick Superman's ass. Goku as a regular super saiyan can lift 10 000 000 000 tons. Superman can lift 2 billion tons, so it will take GOku to go to Super Saiyan 2 or 3 to beat Superman, but seeig as Superman's no Buu, Goku would go to Super Saiyan 2. Goku's ability to turn Super Saiyan and his instant transmission will allow to dodge Superman's blows. Superman will punch Goku in the face but his blows will be to weak to a Super Saiyan to damage them. Goku will then grab Superman by the neck and drag his face against the rocks. Superman will be out of energy an then Goku will punch him till he bleeds. Superman is just lying on the ground unconscious and Goku will wrap it up with a mega ultra kamehameha to the body. Superman's inability to withstand the energy will ultimately take his life. Superman's body won't disappear but he will be lying on the floor unconscious with blood dripping. Goku woudl smile and say, "I'll bring him back and train with him!"