The most important aspect of the game is to continually upgrade your assets whenever possible, when you begin playing focus on upgrading all the buildings around your mansion first before moving over to the city and upgrading what you have there. The order in which you upgrade the buildings around your mansion does not matter.
Resources are needed in order to be able to upgrade your city and mansion. There are 4 different types of Resources, they are cash, oil, steel, and wood and they can be obtained in the World Map by collecting them regularly from Resource Trucks and Buildings that produce them. Resources can also be earned in PvE Battles and Neutral Gang Battles, the amount of resources you recieve in Neutral Gang Battles will depend on your opponents Gang's Level, the higher their level the more resources you will get. You can also purchase Resources from the Family Shop and from the other City's Shops.