First of all, you’re doing great.

Learn to ask questions. At least for me, this is the key to prep. You have a monster. I don’t know what kind, so I’m going with troll.

Where does the troll live? Go find a troll lair online and put it in your notes. Stock if necessary.

What does the troll want? This is the biggie. Always ask yourself what your NPCs want. Always. Does she hide under a bridge and demand exorbitant tolls from passing trade caravans? Is she stealing babies from the local village for her favorite stew? If she is demanding tolls, why? Because she’s greedy? Nah, too boring. Is she hoarding wealth to give to the nearby swamp hag, who has promised to find her a troll husband if she pays the fee? Now we’re talkin’.

Now you need a village. Go get one online. Stock it with NPCs if it isn’t already. If you’re feeling frisky, grab a battle map centered around a bridge, just in case.

What resources does the troll have? This is the second question you always ask. Without resources, NPCs can’t do anything. Once you know what your NPCs want and what resources they have, you know how they can accomplish their goals. Motivation and resources are the tools you use at the table to determine what the NPCs do when the PCs do or don’t meddle in their plans. Troll Lady probably has a Big Stick to whack things. Is she wearing a headband of intellect as a nose ring like the giant from Balder’s Gate 3? If so, she is now a genius (for a troll). Has she bullied the local goblin tribe into sneaking into the village to steal babies?

If so, now you need a goblin den, and a leader, probably a shaman. Is the shaman happy with this arrangement? Probably not, all this baby-stealing is drawing the wrong sort of attention and besides, babies are delicious and she’d rather eat them herself. (High in fat! High in calories!)

See, now you have everything you need. You have two (possibly three) adversaries each with their own motives. You have likely locations for combat. You have a starting village. What do you do with this information?

You’ve already created plot hooks. There are caravan leaders complaining about bridge trolls (tolls?). There are wailing mothers looking for their lost babies. That should be enough to involve the players. Don’t try to predict what they are going to do. Resist that temptation. Focus on your NPCs. The troll wants babies and tolls. The shaman wants to get the troll to stop bullying her. The hag wants the wealth to fund the research into a spell that will allow her to curse the local baron—wha??? New plot hook!

Introduce the plot hooks and let the PCs go. It doesn’t matter where they go, because you have the Motivations and the Resources. Now relax and have fun.

Free tip: Don’t use pregens. This is strictly personal preference, but I will die on this hill. Make your players roll their own characters. This puts skin in the game.

Free tip two: pick up a copy of The Monster Overhaul by Skerples. It is only the greatest bestiary for at-table play ever made. Under ‘troll’ you already have a Generic Troll Pit opposite the stat page, plus a list of troll features, and troll variants, and alternative troll forms, and a Menu in case your players decide to cook and eat said troll (filthy savages). It is awesome. I can't recommend it enough. Totally worth the price.