Short answer: Lower your debt to asset ratio (have more assets than debts), have positive cashflow, and strong profit margins. Airline age also plays a small role.
Long answer: It’s complicated.
You will not get to AAA credit rating in a short amount of time, in fact it could take you several weeks to get into the B’s.
There is a spreadsheet available that you can download to see how the credit rating is calculated for your airline. See here: https://docs.google.com/spreadsheets/d/1HThP4AyS0zw6ouolBKyKzRD3vuo0tnL0gaIB28pcNu0/edit?usp=sharing
Simply having more assets than debts is not enough in and of itself, if you do not have a positive cashflow, it will severely impact your credit rating.