These are the things that make this game "this game". Make errors, go bankrupt and learn from it. There are several strategy's one can follow to run a successfully airline.
Some players go for the big birds while others take the slow proces of smaller planes and higher frequencies and additional slot costs, but a have a stronger position in market share and are less vulnerable to higher fuel prices.
for now, stick to 1 fleet type but get 5 of them as soon as possible. A debt is not something to worry about to much as long as you can pay the bank.