Mainly by participating to tournaments: the further you progressed in the tournament, the more tournament earnings you get. This way all the players earn something, but you get the biggest part if your player gets first.
Having a sponsor can also grant you lots of cash.
If you want to make bigger investments, you can convert Tokens into Cash.
Lastly, you can get some each day with your daily rewards.

With the Pro Team's players you can earn cash at the end of the season (Percentage of their prize money depends on your Assistant Manager).