About The ADND 2nd Edition monster combat!?

Hello, Am new to AD&D and I was given books for my son 2nd edition books and my 13 yr old son wants me to be the DM and am stuck on trying to figure out how to use the THACO, Hit dice and Armor cl in combat for the monsters. What is and How do I use Hit dice, or THACO or Armor Cl in monster battle, (Not player character sequence combat) need the sequence for monsters, what they mean or How to use it?. Thank You!