Due to the nature of tabletop RPGs, a campaign will often start with a random group of strangers. Here are some ways to get them adventuring together, without resorting to the old Tavern trope.
Walking through the alleys of the docks district of town, you hear an old, mad beggar calling out for alms. He claims to be a god, cast out from heaven and stripped of his powers. The party passes, tossing a few coppers to him. In thanking them, the madman refers to incidents in their childhood or distant past which would have been all but impossible for him to know.