I'm not seeing anything in the PHB that explains how fighting from on-top a mount works. How does fighting from on top of a mount work while the mount also has attacks?
Mounted combat rules appear on page 46 of the Dungeon Master's Guide. You and the mount share the round's actions, so for example you can make the mount's move, followed by your attack as a standard action.
You cannot attack in the same round that your mount attacks, unless the mount has a special ability that allows it to do so, or they use different actions. For example, if you cast a spell that takes a minor action, you can still use your mount's Trample attack as a standard action.
If you have the Mounted Combat feat, you can use any special mount abilities the mount has, such as the warhorse's "Charger" ability.