How should my Warlock player use their familiar during battles?

One of my players has a Warlock character and is using Pact of the Chain for their Pact Boon. They most often cast a pseudodragon as their familiar, but neither of us are sure about how they should be using the pseudodragon during fights. Does the familiar get its own turn in the turn order? Or can the familiar only act when the Warlock gives up their action slot? I read the PHB content, but I’m still unclear how/if the pseudodragon should be used in fights.