When combat begins, you roll dexterity or intelligence against the enemies; the highest roll determines which side goes first. Thereafter, the sides take turns acting together. If one side surprises the other, they have advantage on their initiative rolls. If you have time to prepare for an upcoming battle, you can roll charisma against the enemies to give your side advantage on initiative rolls.
Each combatant gets one action per turn, which could be an attack, move, using equipment, or just about anything else. You can move and make a melee attack with one action.
During battle, you can spend your action to help an ally with their melee attack or defense against a melee attack by using your action to assist them. You must be close to your ally to accomplish this. The one exception to this is charisma, which can be used to help any nearby ally.
You can roll charisma to taunt an enemy, forcing it to attack you until another character attacks it, or misdirect it, preventing it from attacking you until you attack it. If your team outnumbers all remaining minions, you can roll charisma against them to intimidate them into fleeing if they're able or surrendering if not.