Some things you need to do:-
Choose your planes AFTER you have looked at the routes available to you. Use the ROUTE PLANNING button.
Little planes make small but steady profits, 3 of them can make enough to pay for themselves and the base operations, even if one of them doesn't do too well. If you survive a few months you can change to new routes that may pay better.
Don't borrow until you are making money, and when you have saved half the price of your next investment it may be worth borrowing a little.
Keep your planes busy. That means choosing a base that has almost unlimited flying hours. 4 to 6 flights a day for small planes because turn around times are short, small jets only get a couple a day in, and the 150 seaters are often scheduled out one day, back the next, to get best ticket prices (daytime take-offs).
Make the stop over time twice the minimum, then your planes won't be late for their next flight.
Make the gap between flights just 5 or ten minutes, depending on the flight time.
Most of all press all the available buttons to get an understanding of what's available to operate your airline.