Re: Unhappy with my own world building skills.
Two things about campaigns, and some other stuff after,
One, don't prep plots, prep situations. A plot can fail at any point that the players go somewhere other than what the plot called for. Prepping a situation however, doesn't care what the players do. Example, you might have wanted some thugs to be stopped by the players and that the thugs would have a clue on them, but what happens if the players let the thugs escape? Or a player calls the guard and they let the guards handle the bodies once the fight is over? Could you avoid these things? Sure, but it might seem akward and forced depending on what the players do.
Two, the three clue rule. Any time you want the players to find something, make at least three completely different ways to find it. Using the example above, the clue from the thugs was supposed to indicate that a necromancer is about to attack a city in the north. If the players didn't get the clue from the thugs, they might hear a rumor about how the local lizardmen have become scarce, believed to be working for an evil wizard in the north, or they might stumble across a smashed cart that has necromantic artifacts and the manifest says they are headed to a city in the north. With all the ways of telling the players the bad guy is up north, you can easily play the NPCs true to their nature without having to compromise them just to give the players a clue. It also helps when the rogue drags the party along to find rumors before they have a chance to meet the thugs, you can just flow with it and give them the rumor clue instead.
As for worldbuilding, you can design the world first, or you can design based on your needs. The difference is a matter of what is fits you best. If you're terrible at improvising, then building the world first might be what you need, but if you improvise, you can just add details as they are appropriate. (Just be sure to write it all down to maintain consistency).