Combat is a large chapter in the main Gaia book, so I will try and keep it simple and inform you of the most basic things you need to know about combat. First you must roll for initiative, rolling a 1d100 and adding the weapon speed, any initiative bonus from your class and your agility modifier, open rolls and fumbles (Rolls between 90-100 and 1-3 respectively) still apply. If you beat any combatant by 150 you surprise them, causing them to have a -90 negative to their dodge and block rolls. In combat you only get a set number of actions, based on the combined total of your Dexterity and Agility. Here is the chart from the book to inform you of how many actions you may take per round.
In combat however you can only do each action once. You can move once and only attack once and use physical abilities once. Here are a list of actions, both active and passive.