As you look around the World Map, you will see areas named Cash, Wood, Steel, or Oil.  They have Resource levels between 1 and 7.  You can click on the Resource you wish to gather and send your Troops to collect it, but be aware of how much your Troops can carry.  For example, if you find a Cash Resource Level 6, it will offer 298,000 Cash.  If your Troops can only hold 200,000, you will collect the Resource until you reach that amount and then your Troops will stop gathering and return to your City