The safest way to earn money is by upgrading your buildings in your town that will earn you money. That way you ensure a regular income.
Successful attacks on the Wild Bunch Gang will always earn you a good amount of money.
Trade businesses in the darker places on the map ensure you a nice but risky additional income.
To make a quick buck, attack other players in PvP mode via the button in the upper right on the menu and win the fight.