Initiative is determined in order from the lowest number to the highest.
Each character has the Initiative of d20 roll minus dexterity modifier (maybe without ability modifier, but then what should mimic a better response time?)
When it is your turn to play you choose an action.
Common actions include:
Each action has a specific point cost – like, 10 initiative.
When you choose an action your initiative grows buy the action’s point cost.
After the current score has chosen their tasks and updated their initiative scores, all initiative scores are reduced until the next person reach his turn.
When the time for the next action comes, the last action that was chosen resolves.