How do you get in a war?

From the wiki

Where can I find clan wars?

If you are in a clan, you can access clan wars at any time via the clan wars button on left bottom corner on your screen (the button with two swords crossed). From here, new clan wars can be started by clan leaders and co-leaders, and you can participate in or spectate any clan wars that are in progress. If a clan war is in progress, pressing this button will take you directly to the map of the war zone.

How does one start a clan war?

Leaders and co-leaders can take their clan to war by opening the clan war screen and pressing the “Start War” button. This will start the search for an opponent clan, and you will be able to see the size of the clan war you are being matched for (e.g. 15 vs 15). The clan war will start immediately after an opponent clan has been found.

Finding an opponent clan can take up to a few hours, but there is no need to wait in the clan wars screen. You can close the screen with the red “X” button and you will be notified when an opposing clan has been found.

How do I participate in clan wars?

To participate in a clan war, you must be a part of a clan when a leader or co-leader presses the “Start War” button. If you join the clan after this point, you will become a spectator and will not be able to actively participate in the clan war.