Making a dungeon is a basic part of almost any D&D adventure, especially because the game itself stretches the definition so far, it's joints will never recover. it now means "a probably enclosed area where challenges occur". Even so, there are a few good points to keep in mind that will make your dungeons far more memorable than just having another run-of-the-mill encounter... area.

Decide on a theme: Before you start designing your dungeon, you need to decide on a theme. This will help you determine the type of monsters, traps, and puzzles that will be encountered in the dungeon. Some popular themes include underground caves, ancient ruins, and haunted castles.

Create a map: Once you have a theme in mind, it's time to create a map for your dungeon. This can be done on graph paper, a whiteboard, or using online tools. Start with the main entrance and work your way through the various rooms and corridors of the dungeon. Don't forget to include any secret passages or hidden rooms!

Populate the dungeon: Once you have a map, it's time to populate the dungeon with monsters, traps, and puzzles. Make sure to balance the difficulty level of encounters to match the level of your players. We'll go over making your own monsters later, but for now the monster manual has enough options.

Add treasure: What's a dungeon crawl without treasure? Make sure to include treasure chests, secret stashes, and other rewards for your players to find. Consider making some of the treasure cursed or booby-trapped to add an extra layer of challenge.

Test run: Before running your players through the dungeon, do a test run yourself. This will help you identify any issues with the map or encounters and make adjustments as necessary.

We'll go into depth in each of the points above, but for now, the best way to learn anything is to get your hands dirty and try it. Make a dungeon! It will probably be bad if it's your first attempt, but the more you do it, the better it will get.