All players can perform an attack using their army. See full guide on attack rules.
To help defend your town from enemy attacks or to perform an attack yourself you will need to train units. Every unit has its own strenghts. You can choose to train cheap units and use strenght in numbers or hire more expensive units with higher attack and defense stats.

The stats of the units is displayed in the bar beneath the unit image:

The above sample show the stats of the cheapest unit. It has one attack point and one defense point.

To train or hire units use the menu to go to the army page.

For the tutorial purpose you don't need to train a complete army. The tutorial area is here for new players to learn the basic game mechanics.
Tip 1: Train units to protect your town, but don't forget to build defensive structures. They will help you alot.

Tip 2: When attacking with only bikers, your travel time will be halved.

Tip 3: Don't attack to soon, the enemy might have a bigger army. Always try to gather information using the special operations.
Visit the full guide for the complete section about Units