Buying aircraft is one of the most important actions of all, but first, you need to think about the type of aircraft you want to buy.
Airline Mogul has a large number of aircraft in-game, all modeled as realistic as possible. You can always find the aircraft you need, but you'll still have to consider your options. Airplanes have various different statistics, but here are the ones you should be paying much attention to when you're new to the game.
The important factors
Obviously, it's very important to look at your bank balance. If you have little money, you won't have to even consider buying a Boeing or an Airbus jet, because you won't be able to pay for it. You can also choose to wait until you do have the money, but in the meantime, your airline won't improve at all. Usually, small airlines are better off starting with small aircraft. If you don't have enough money up front, you can place a broker request, either public or private. When somebody brokers a plane to you, you only have to pay 35% right away, and the other 65% when the plane arrives. Quite often, though, you end up paying more in the end! Remember though, you cannot go into negative values while buying aircraft, but you CAN while buying gates. So make sure to buy gates after you buy planes if you are planning on getting a certain one that may cost all of your money!
Airplanes can roughly be divided into two categories: jets and turboprops. The difference between them can be seen by looking at the airplane's speed: anything above 400kts will usually mean you're dealing with a jet. The advantage of jets is, as noted, their speed, which means you can fly more flights in 24 hours. However, fuel costs will be much higher than for turboprops.
The maximum range of an aircraft determines how far it can fly. If your airline will have to make longer flights, you'll need an aircraft capable of doing that. However, if your airline is, say, based in Europe, the maximum flight length will mostly be about 1500 miles, so you won't need a long-range aircraft.
Also notice that the range of an aircraft may vary with the different engines that you can choose when buying it. These changes can amount to several thousand miles.
The more seats, the more passengers fit in your plane, but that's nothing new. It might still be an important factor in deciding which airplane you want. If you're planning on transporting large amounts of passengers, you need a bigger plane. But do remember that the more seats you want, the higher the costs will be.
Some aircraft will have the option to choose a certain engine. Aircraft with different engines can fly different ranges, have different speeds, and might be able to carry different passengers. They also cost different amounts. Make sure you are not choosing an engine that will cause the aircraft to perform differently from what you wanted when buying!
As noted, turboprops use less fuel than jets. Early jets are usually turbojets which have a significantly higher fuel consumption than the more modern turbofans. Over time fuel consumtion generally decreases as better technology is developed.
To make a final decision, you can go to the Buy Aircraft page to view all available airplanes. Once you've made your choice, order the plane and have fun with it. Remember that the more time you take to plan things properly, the better it'll work out.