As a cargo airline. You need and only need to operate at routes than has more than 400 combined cargo ratings and i advise you to begin playing at the mid 1960's as you'll get more cash to start with because cargo planes are expensive and not spammable.
After starting, try to find the most profitable monopoly routes you can ever find always stay out at competition cargo routes unless if it goes to a major city to another major city. But, this is not to be worried about as most decent routes are not occupied by another airline becuase of unpopularity among choices.