Hell, It's About Time!
To train your first Space Marines, you will need to raise the Asgard to level 2, repair the Armor Barracks, rebuild the Barracks, extend Armor Barracks to the second level, and then research 'Combat Simulations' (at Armor Barracks).
Raising an army and researching new technologies is an expensive endeavor. Soldiers demand food, the Barracks must be upgraded to unlock new units and Stargate has to be repaired to allow for troop transportation. If you want to attack somebody, first go to MAP (at the top menu). Scroll the map, select a target, scroll down the Profile page, and then click 'Go to Stargate' inside the 'Send Troops' column.
At the Stargate, select the action you want your troops to perform (reinforce, raid), enter the number of units you want to assign in each case below the unit's portrait, and finally hit 'Send Troops'. An army stationed in a colony will defend it automatically as soon as it is attacked. Defenders will use the 'Counter-attack' attribute to inflict damage to attackers.
TIP: At each 4 levels of Stargate, you will get 1 additional deployment queue.
Target acquired, moving on...
Tell me about taxes again