File City - Digimon World 1
File City is the very first location you visit in Digimon World and it's the main hub for all your future adventures. There are two Digimon that you can recruit in this town but they won't become available until later and they're triggered by talking with Jijimon. Once you reach the Prosperity Points required and talk with Jijimon - leave the building and you will be attacked.
I recommend you avoid talking with Jijimon until you're ready to fight Greymon or Airdramon. For Greymon any Champion Digimon will work, you should probably have an Ultimate Digimon for Airdramon though.
Greymon
Requirement: 15+ Prosperity Points
Speak with Jijimon once you have 15+ Prosperity Points and then exit the building, Greymon will attack you.
Airdramon
Requirement: 50+ Prosperity Points
Talk to Jijimon inside of his building, if you have 50+ Prosperity Points you'll trigger a fight with Airdramon when you go outside. Beating Airdramon will unlock the final dungeon of the game, Mount Infinity.