There are many ways to collect Resources throughout the game.  In the World Map, you can gather Resources from designated areas, defeat Neutral Gangs, or attack other Cities and plunder their Resources.  In the Street Map, you can open the Storybook mode for chances to gain Resources and Items.  You should also regularly collect them from your Buildings or Resource Trucks.  Lastly, you can always purchase Resources in the Shop or in your Family Store.