Basically, you more territories you own, the more income you receive.
That might make you think, well this is simple, I just have to quickly expand, not exactly.

The best method I've found to starting my approach to making income early game and late game has always been having a Squad travel to enemy territories and kill hordes. This generates and enormous amount of wealth, mid-game a tremendous amount.

• Step 1: Choose either the default location (which is already an owned territory), or pick a point on the map to take over a territory in an area of your desire. It doesn't matter too much, unless you choose a overridden territory in which case you'll have to defend the territory immediately.
  
• Step 2: Once you've chosen a territory you can dispatch your Truck from the call-down menu, this can only be done for your first Truck. Once your Truck is stationed you can use it to build a Medic Station, Ammo Station, and Garrisons.

- Ammo Stations can be used to refill Troop ammo. This can be ideal when you have a forward base with units stationed to defend it, they'll automatically refill their ammo when in the vicinity.
- Medic Stations can also be used to refill Troop health in the same situation.
- Garrisons are very important. Garrisons increase your MAX population so you can recruit more units.

• You may not need to build the Ammo/Medic Stations right away, depending on where you start. So don't get used to a certain build order, only build what you need when funds are limited.
  
• Remember: You can only build one CERC HQ, this will be your base until it's either destroyed, or your destroy it. The CERC HQ is where you'll purchase upgrades.
  
• Step 3: Your first Engineer, you begin with an Engineer already in Group 1. You can use this Engineer to station at your Main Base. Engineers can build all the Structures for your base, including Walls, Turrets, Barracks and other complimentary facilities.

- Engineers, I like to station 1 Engineer at each base. I also like to Assign them to their own Group, selecting your Engineer you can press CTRL+123456789 (to assign a group), once you have them placed in a Group you can shrink their Icons using the downsize arrow in the Group box. This will make their icons appear as a gear. If you have 1 Engineer at each base you can quickly double-click on their Icon and it'll immediately bring you to that base on the map. I put all my Engineers in the same group to save space on the UI.

- Bring an Engineer, or Build an Engineer? When clearing a Zone to establish a Base, I find it personally easier to clear the Zone, build a Base Truck or have one on standby, and then have it drive to the place I wish to establish my base and from there create an Engineer from the Truck after it's stationed.

• Step 4: Have your Engineer build a Barracks so you can begin buying Units, you'll need a Generator to power the Barracks (you can use Pylons to expand the distance of the power from your Generator to reach Gates, Barracks and other buildings or defences that require Power). Once the Barracks is in place you can hire some units. At the beginning you can make it simple and just hire Assault units. I typically hire the amount of units that will fill the Helicopter you start with, without bringing your Commander with you, or the Engineer you started with.
  
• Step 5: Using your Squad of purchased units, you can travel to Enemy Territories either by foot (if you spawned in hostile territory) or by Heli if you have to travel a fair distance.

- Enemy Contact, your Units will fire from the Helicopter if in the air so there's no need to risk your Units in conflict. Be aware however that certain Enemy Units can spit at your Helicopter or even jump onto it and damage it. Helicopters are fragile so don't go deep into Enemy Territory if you plan on flying overhead large strong hordes. If on the ground, you can put your units in a variety of different formations to strengthen your firing line. I personally use a tight V formations and O formation quite often. I use Double Line formations for firing lines at defensive positions in bases. If you double click, your units will run to a position instead of walk. Be careful with this as you can run around alleys unknowingly that have hordes in them, they'll immediately flank you.

- Enemy Hives, you'll see on your map small, medium, or large pink spots. These spots are hives, when you have a visual they'll change their shape according to the size of hive. When a territory is overridden with infected, you can purge it by finding all the Hives and destroying them. If the area has a decent population still, the Hives shouldn't be too hard to take care of. You'll probably want a Squad to 10 at the beginning. But feel free to chip away at enemies for income.

• Step 6: Once you've cleared a territory, or made a relatively safe position (you do not need to fully clear and area to establish a base), you can buy a Base Truck and have it drive to the position you've secured. Once there, station it. Build an Engineer, build a Barracks and Generator and Garrisons. If this is a forward base you'll also want a Ammo and Med station to keep troops stocked. After that you can enclose the perimeter with a Fence, don't forget to add a Gate so you can open and close it as needed (or set to Automatic so it opens when units are close). Once you Fences are built, you can double click on the Fence to select all, from here you can Upgrade them to the next level.
  
• Step 7: Make defensive units. There's no point in putting this units in a Group unless you want to for your own playstyle. I like to have 5 Defenders early game, and 10-15 Defenders mid-game. Just set the rally point on your Barracks to a position and hire some Units. Once they're there you can select them and put them in a desired formation at a good area to defend your base. You can also have them Patrol a certain area, although I find with the pathfinding issues it creates more harm than good to have them patrolling.
  
• Step 8: From this point on you can continue securing territories and expanding your protected Zones.

When making units you can assign their Group before purchasing them.
- If you didn't assign a group, you can find them easily at their rally point, select them, and press CTRL+123456789 to put them in your desired group.
- I typically have 1-2 Attacking Squads with about 20 Units each. This Squads are always in enemy territories raking in the income. Income from killing enemies is much more than territory income. But you need territories to make more Garrisons, thus, make more units.

- Have medics be in SAFE Fire-Mode to not fire at enemies. If they're in SAFE they will heal allies even during combat.

- Do not forget to use abilities. When you hire specific units they have very useful abilities. Heavy can have increased Fire Rate and Bipods to take control of a choke point. This also applies to late game units. Always remember to use their abilities when in a pinch.

- Use Airstrikes and Ammo drops (You will need a Radioman in your Group). These are incredibly useful for your Zone Clearing Squads. If you're getting overwhelmed in an infested Zone an Airstrike can give you some breathing room. If you're locking down an area for quite some time, Ammo will be crucial to continue the fight. You can also it down right beside your men.