It would depend on how its done.
If its an established Superman film with no origin, then i'd open it with superman in the middle of a fight with metallo. After 10 mins or so of fighting, Superman defeats him. Lex would be watching from Lex-corp and is enraged. Afterall, metallo was his answer to Superman.
Brainiac arrives later, makes contact with lex and.......you get the idea.
If its an origin, then Have superman make his first appearence in metropolis doing some huge rescue with lots of media attention, etc etc.
Lex is angered because the spotlight is taken off of him and placed on Superman instead. Lex gets to work on a plan to get rid of Supes.
First its intergang. That fails. Next Metallo, again it fails but he does give superman a pretty good fight.
Then, as luck would have it, Brainiac arrives. He makes contact with Lex and a partnership is formed.......