The first thing that happens in a combat is that each character rolls Initiative. This means that the first time a PC decides to pull out a gun or attempts to pummel someone, before resolving that, the first thing the GM does is say: "Roll Initiative!"
The Initiative roll for virtually all characters is (Speed × 2) + 2d6. All PCs and named NPCs should have unique Initiative scores, but groups of "goons" – similar enemies – can all act on the same Initiative, to simplify bookkeeping. Characters act in Initiative Order, form the highest score to the lowest. As Initiative "collisions" will happen, characters with the same Initiative act simultaneously: i.e. two characters with Initiative 14 shooting at each other will both get shot at. Even if one of them is shot and killed by the other, his shot still goes off.
Note: Characters with the Reposition Technique have the option of rerolling their Initiative between Combat Rounds, or keeping their last Initiative. Characters without Reposition are stuck with their Initiative result for the duration of the combat. In other words, by default, Initiative is not rerolled between Combat Rounds, unless a character has the Technique that lets them do that.