You want to make a dungeon. You want it to be the centerpiece of play. You want dungeon to be the game. But how do you start on building a big dungeon? I'm going to walk you through the easiest steps to get up and running.
This is the basis for everything to come. Have a one sentence description of what the dungeon is - Something that might indicate past uses and what might lie inside. Examples:
There is no mandate to choose a theme and stick to it unerringly. You may change and revise it if you come up with better tweaks. You are also not bound by its letter; take the theme as loosely as you like.
Decide on the scope of your dungeon. Do you want a years long-campaign or a one shot? Estimate how many sessions you want out of it, and assume 5-7 rooms/session. Then divide into levels, more rather than less.
Come up with lists of monsters for different levels of the dungeon.
You should think about how big of a list you need for each level. This will depend on your stocking procedure. If you plan to roll inhabitants randomly into rooms, calculate how many monsters you will need on average for your dungeon floor. Create a list of that size for that level. Double the number to incorporate random encounters.
This mimicks old D&D stocking procedures. You can get more creative with how monsters move around, but for a first dungeon, this is a perfectly fine way to do it.
Jot down a list of trap ideas appropriate to your theme; it does not need to be fully fleshed out and can be filled in when stocking. Take 'traps' loosely: you can have 'environmental hazards' as well, just not a thing that is killable like a monster.
Special rooms have something weird but not harmful. Again, keep a list for the stocking and come up with it on the fly.
You can draw your own, but it's easier to pull a random map from Donjon.
Roll the dice!