some considerations from me:
1. Airport Location
Entering late into the game makes it difficult to simply allocate your headquarter to one of the big hubs, because most slots and attractive routes may have been taken (when entering a new game from the first day, you may however choose a big airport hub). This is not attractive because the most important thing is, to find free routes without competition to generate good cash flow. When entering a game at a later stage like right now, you may want to choose a large airport, which is has no airline based there and still offers attractive and free routes.
2. Fleet Strategy
Due to the lack of money at the beginning, you may want to consider only smaller aircrafts in order to keep cash flow controllable. However, there have been a few successful strategies been implemented going for big aircrafts and even long-haul flights. I would consider these strategies only in the case of a sustainable advantage that it occurent from the first day, like free routes or niche market entry (only first and business class for instance). The more common way would be to choose one smaller aircraft and deliver routes with it. You will probably only be able to lease this first aircraft. Remember that you pay four lease-rates upfront when ordering the plane. That means that you earnings in the first four months will be distorted to the lack of these lease-payments not showing up yet. So first excitement about generated cash-flow may end in a hangover four months later. Also taxes will kick in after a few months and further reducing your earnings and cash-flow. Also consider the maintenance costs of aircrafts and the fuel costs, which may be decisive over profit or loss during operation. Generally, a common fleet will keep maintenance costs low.
3. Route Strategy
Generally it is of absolute importance to have your airplane in the air generating money. When starting a new routes, try to find free routes. Look for the pax demand and which aircraft may fit that purpose. You may want to choose a route that has higher demand than your airplane capacity, making it easier to reach high load-factors. Further, with small aircraft, consider short routes. You may fly from your airport to airport X and connect further from there to airport Y. If the routes are short enough, your plane will go out at 6.00am and be back from those 4 legs at 02.00pm - enabling you to fly another four legs like that in the evening. This way, you should get your plane to 8 legs per day on short routes. Concerning the load factors, you can stick with the initial prices and see how your load factor will move, once you lower or higher the prices. Don't make too robust price moves. If you have 8 legs per day you should generate a load factor of above 70% within the first 4 weeks. That should keep your airline solvent.
Don't rush too soon at expansion. Your first four months of operation will be free of lease-payments, easily making expansion look very attractive. However, calculate how your monthly results will be including those payments and consider then only expansion. Due to cash, you may again only consider leasing another plane. When purchasing or leasing a plane, make sure you get the C-Check accomplished before you get into operations. Flying a first airplane that will have C-Check within 3 months e.g. may kill your cash-flow and threaten your solvency. Prefer to wait four weeks for a used plane, but have that check performed.
There are further aspects like marketing and pricing etc. But for a start, I hope this may help you a little with your decisions. Again, these are considerations, I view as important when entering a mature market as a start-up airline. Totally different scenario when going it at zero.