The first picks on each team are captains. In ranked it'll be the person with the highest elo; in normals it's decided normally.
Each captain gets three bans, alternating back and forth between the two. Captain A bans one, then Captain B, then A, blah blah blah.
I suggest playing a game or two of normal drafts before going into ranked just so you're used to it and aren't as intimidated during champ select.