I like the "iceberg" approach to designing content: you should know quite a bit more about everything then you reveal to your players. So when laying out a dungeon, consider how it was formed or built. If you were building a fortress, how would you lay out the rooms? If you decide that a dragon took up residence in a particular cavern years ago, describe that location as scorched. Even if the players never learn about the dragon, it adds flavor.
Try to give each dungeon something unique. This one was tunneled by a giant worm. That one isn't a dungeon, but a tower. This one was built to hold pirate treasure, and is filled with hidden rooms.
As a first time DM with experienced players, I'd advise embracing the philosophy that this world exists independent of the players, and thus isn't finally tuned to their needs. Sometimes a village is being attacked by a dragon when you're only 3rd level. Sometimes the ultimate magic ring is just in the hands of a hobbit. That way you can offload some of the encounter balancing on the players. If every encounter is tailored to the party and they die, that's your fault for throwing something too hard at them. If you offer them all different difficulties and they chose to pursue the titan, that's there own poor risk management. Just make sure not to be stingy on escape opportunities.
Observe your players. What content do they perk up for? If their attention is flagging during plot exposition, cut it short and present more information later. If they're having fun running errands in town, don't force them to move on. Besides being crucial for pacing, you can start to learn what makes your players tick. Is it love of gold? Now you know how to give them meaningful options in the story, but still be fairly confident they'll choose the choice you like.
Finally, while you're still building up instincts for presenting content, try to build some "outs" for yourself. Is there a big fight you're worried about balancing? Describe a large bell in the room. If the fight goes well, that was flavor. If the fight is too easy, a foe can ring it for reinforcements. But make sure to give clues beforehand. The wolf howl doesn't mean wolves have to attack, but if they appear after a battle that turned out easy with no warning, it feels like the DM is cheating. On a similar note, don't be afraid to pull content. If the players seem to be growing bored with a dungeon, the kobolds you planned in the next room can vanish. Don't go overboard, don't fine tune the number of foes so the party always just wins. But if you accidentally made a dungeon way too tough, you can tone down the later rooms; if the players aren't having fun with something, you can find ways to end that content early.