You can get 1000 yen every 30 minutes once you are a grade-12, this is a good way to get money when you start
Auction house is a great way to get money as you can sell what you own, alot of the stuff from player-ran stores that open periodically can be sold in the auction house for a higher price so you can easily profit. Faculty, KPD, EMS, and stores are also normally hiring where u can get money for being a teacher, police officer, retail worker, doctor, and more. These are the best ways to get money when you begin but there are so many more different ways to get money as you proceed further, such as towers, estates, custom items and more.