Searching
Looking for missions is an easy process, since all Mission Givers for Main or Side Missions have a blue exclamation mark over their head and generate a blue beacon. After a mission is started the Mission Giver will have a blue question mark over them until the quest is completed. Challenges also give off a beacon as long as you have the correct character for that Challenge. Initially the beacon is yellow to represent the first level of difficulty, which is easy. As you complete a challenge the beacon will change to orange for medium and red for hard. When all three levels of difficulty are completed it will turn grey.
A lot of the guesswork is taken out of searching due to the green compass arrow built into the game. When in doubt, go into the mission log and make sure to activate a mission to get the helpful arrow to appear.
Show Me the Money
A lot of the destructible environment as well as enemies will drop coins, however you must actually run over the coins to collect them. It is easy to overlook this fact, so make sure not to miss out. If you are running low on funds, try completing some challenges on the higher levels or simply find a spot with a lot of destructible objects to smash and let them regenerate so you can do it again.