To create an army you need 3 units of the same type in 1 square and press Y to make an army out of them.
Choose any civilization, in the first 5 turns the game will automaticly make you 1 warrior unit.
Press X to have them guard your capital and wait 5 more turns to recieve another warrior unit, wait another 5 turns to recieve 3rd unit and press Y to create an army.
Bleep bloop, achievement unlocked. :)
Note: your units don't have to be in your castle, they can be anywhere on the map aslong there are 3 of the same type in 1 square.
This video will show you in the first 2 minutes how to do so.