Firstly make your stadium optimum capacity of building and select a sponsor, for more information look sponsor and stadium expansion pages.
But key point is complete the cheaper buildin first and sitting capacity
In a long period of time main earning comes from transfer
For example; Buy a player with tackle is about 4 or 5 and in fifteen week just trained him tackle and when he took 2 balls or 3 ball for tackle you probably sell the player for a big money. I sell 6-7 player by this way in a season and i gain approximately 1.000.000 euro extra money.