Do I need to build an army to attack in a clan war?

Yes. Your attacking army must be trained and prepared in your village just like you’d do for a normal multiplayer attack. It is up to you if you would rather find a good target for your regular army composition, or if you would instead train an unusual army to take out a specific base with a certain weakness. One thing is for certain, however: there are no second chances. Remember to check that your Heroes are awake, that your spells are ready and that you have your Clan Castle full before using one of your precious clan war attacks.