You can pick up a battle with another village as soon as you have your barracks and one troop ready.
Go to the Battle tab, select the type, and quantity of troops to assemble your army, select a target from the list, and hit the attack button when ready. After the attack button is hit, and once the transaction is confirmed, the battle will take place.
Everytime you move troops through the portal there is an associated cost in Crystal (you can lower it by leveling up your portal to get to a maximum 40% bonus). However it’s smart to keep this in mind when selecting a target since the costs of moving the troops + the cost of the troops lost in combat vs. the resources to be looted might make the attack a bad idea. Unless you’re after that village then no expense should be spared to smash that quarrel.
The targets you see on your target list are villages with 25% ELO below and 200% ELO above yours. If you don't see any targets it probably means your ELO is either too low or too high.
To check your results, and the results of your past battles click on the Battle History tab.
If an attack is successful, up to 50% of the resources on the target village may be taken by the attacking army as loot.
You won't be able to attack another users village if their shield is up, and they won't be able to attack you either if your's i up but keep in mind that shields have a determined duration and a long cooldown.
Another thing to consider is that when you select a target what you’re seeing is a snapshot of that target’s army, and resources that may change from the time you select the attack to the moment you carry on with it so it’s a good practice to always refresh the target list before launching an attack to make sure the outcome will be what you expect.