When combat begins, by default, the heroes go first. Whichever player is ready first goes first (or whoever makes the most sense story-wise), with play proceeding around the table clockwise. Monsters will typically act last, though some monsters may be fast enough to act sooner in the round. A monster (or monster group) should act at the same time each round.
Encourage teamwork, be flexible If players want to strategically switch their turns for one round to facilitate teamwork or execute a tactical plan, the GM can allow this at his/her discretion. This should be done only occasionally, though, to avoid slowing the game down or losing track of who has taken their turn.