The group stage uses three points for a win, one for a draw, nothing for a loss. Each group has four teams. Every team plays three games.
Teams are ranked by points. If sides finish level, the tiebreakers go in this order: goal difference, total goals scored, head-to-head points, head-to-head goal difference, head-to-head goals scored, disciplinary record (cards), FIFA ranking. In practice, most ties get resolved at goal difference or goals scored before anyone has to look up FIFA rankings.
The top two from each group go through automatically to the round of 32. That is 24 teams from 12 groups.
Third place is where it gets more complicated. Eight of the 12 third-placed teams also advance, chosen by comparing their records across all 12 groups using the same tiebreaker criteria. So finishing third does not end your tournament, but it might, depending on how the other third-placed finishers around you did.
Six points from two wins and a loss will almost always be enough to make the cut as a third-placed team. One point from three draws is a much harder position to survive from. The teams that go into the final group game needing a specific result from another group to go through are the ones who will feel the 12-group format most acutely.