You have to go to the Garage immediately after Josef gave you Car Key ( First week of the game ) to meet Mario. Then go to his Villa and straight to his Office. He'll give you an offer to work for him. Accept it and you'll unlock Mario's Questline.
As for meeting Sofia, you have to progress a bit further in Mario's Questline ( Finish the Resumé Minigame ). Go to the Mall right after and you'll meet Sofia, who will invite you on a "date" at the Gas Station. Turns out she's a Cop and she has been watching you helping Mario, thus she want your help to get evidences on Mario in order to bring him down to jail.