I need some help/advice with my new job, about taking control of the system (web, javascript, nodejs, android, brokersms, fcm)

I started a new job in the middle of this month, and my first task is to "domain" this software that was made by the last intern. The software was made to notify all clients that they have to pay for the products/services theyve demanded, by push (android app) and SMS (mobile). Well, its been two weeks now and im totally lost, because i have no experience in web development and i believe this system is built in js/nodejs. There is no one to train or guide me, so that makes me aflict, because ive barely made progress so far. Can anyone give me some advice? where should i begin? how can i take control of this system?