Superman is arrogant, so he'd see the first villain, destroy him, then wonder if everyone is a villain - consider himself the good guy in a world full of horrible people, and try to kill everyone. He'd be stopped.
if superman would go into marvel he would think he can kick everyones ass just cause hes the most popular guy. but afew guys on there would show him hes not the strongest. so hed fly his ass back to DC
1) Meet up with Reed Richards who would proceed to do tests on him.
2) Discover that his powers are actually a result of exposure to kryptonite at an early age which mutated his body on a molecular level as an infant.
3) Spend time with the Fantastic Four as a replacement for the Thing while Ben takes time off to feel sorry for himself.
4) Leave the FF to join the X-Men after it becomes public knowledge that he is a mutant.
5) Spend years with the X-Men while everybody guesses possible scenarios for his REAL origin.
6) Get his own title.
7) Get a second title of his own.
8) Share a title with another popular Marvel hero
9) Have a title where he co-stars with a new Marvel hero every month.
10) Have his whole character retconned after years of "service"