You can pick up a battle with another village as soon as you have one troop ready but it’s better to do so when you’ve at least one bison as well so you can loot resources from the enemy in case your raid is successful.
To start a battle click on the Battle tab in which you’ll see the following:
On the top right indicator you'll see the combined stats of all your troops available. On the top left the time remaining on this wave (check the Wave entry for more information).
Below your top indicators you'll find your available targets. In here you'll see the combined stats of the enemy, the resources available to loot and the terrain in which the battle will take place.
Once you're ready select an enemy from the available targets list and click on prepare attack, you'll see the following:
On the right you'll the stats of the troops you've selected for this attack. You can change the amount of troops selected for each attack by clicking on them and moving the slider.
On the left you'll find the stats of the enemy village including the terrain in which the battle takes place. There are different types of terrains and each will increase or decrease the performance of your troops for that specific battle. Check the Terrain entry for more information.
In the middle you'll find a preview of the battle outcome that includes the expected casualties on your side, the resources you'll be able to loot if you carry out the attack and the transport cost (which varies depending on the size of your army and the level of your Portal).
When ready click on Confirm and attack to carry on!