There are different ways to get new units. 
1) Go to the Shop and buy Booster Packs. 
2) Sacrifice units to level up the Mystery Box and open it. 
3) Finish a Campaign to receive a unit as a reward. 

The Basic Booster contains only Common and Uncommon units equipped with Basic Command Cards. This type of Booster is very cheap, so you can use the unit for composing Evolved Command Cards. But if you're looking for Rare or Mythic Rare units and better Command Cards, you should switch to the Advanced Booster or Premium Booster. For new player I recommend buy Basic Booster and later Enhance Booster. Premium Booster is not worth to buy.

Coins are the Soft Currency of Panzer General Online. You need Coins for purchasing Boosters or for trades in the Marketplace. Coins can be earned by questing, winning Multiplayer matches or through the Daily Reward. 

By using the Composing feature, you are able to add a Command Card from a second unit to a currently selected unit. This second unit must have the same rarity, same faction and must only have one Command Card attached. The second unit will then be DESTROYED after the composing. 

When you add a fourth Command to a unit by using the Composing feature, all three previously existing Commands will be destroyed and replaced by an Evolved Command. Evolved Commands are much stronger and are very flexible Commands. On the down side, they tend to cost a lot of Command Points and require Elite Stars. 

Mystery Boxes are Boosters which can be upgraded by Sacrificing units. There are 5 levels of quality which can be reached. Each level offers a specific reward in terms of units and/or resources. 

You can switch factions by clicking/touching the Company button in the center of the Top Bar. By selecting another faction you'll also switch the playable side of the Campaigns. 

Your army consumes Supply when fighting in a campaign. The amount of Supply depends on the unit in an army, so a Tiger tank will consume much more Supply than a Rifle Team, for example. When you are out of Supply your army will no longer be operational and you will have to refill your Supply.

The easiest way is to use Refill Supply Items. These items can be purchased in the shop or can be rewarded by playing special quests. The Supply is also automatically refilled when reaching a new Experience Level. Another way is to support your friends by solving running challenges. These Support Battles are rewarded with Supply.