first u need to talk to the girl and sheel tell u about her dad
then u have to go to the flower meadow and in the top left corner u will see a path way go into it
there u will fight 2 galactic grunts.
Then they will drop a key use this key to open the door to the valley windworks
destroy team galatic in their. after that u will have to fight mars one of the commanders be carfeull cuz one of her pokemon is level 17
afterr u defeat her team galactic will leave and u can cross the bridge. ironically u dont have to battle the people on the bridge!