How can I play as a dungeon master ?

I already had my first session ever today, we just started the introduction of the story because we were figuring out character sheets all day and reading instructions, my story telling was kinda decent for my first time since I was translating everything to my friend and gf, but when it was time for a battle I realized I didn’t know how to control the monsters, I perfectly understood how the other players fight against monsters but not how monsters fight back (which im supposed to control) because the rule book doesn’t make it that clear and focuses more on players controlling their own characters, can someone please explain how I can control the monsters and how I can improve the story telling and make it funnier for my friends please, today was my first time playing so please try to explain it as simple as you can 😭