The First Thing you should look for when placed in war is the enemy base that has the same number as your war base (AKA Mirror Attacking).
The Second Thing you should look for is the best spot to lure out the clan castle troops. Failing to lure out and kill the clan castle troops in the beginning will cause your attack to fail about 99% of the time. Below are two videos that will help you learn this technique. *Warning* These videos don't have the best audio quality but the information they share is VERY Valuable!
For those with a Town Hall 4, 5 or 6... TH4, TH5, TH6 - How to 3-Star!
For those with a Town Hall 7 (The Mass Dragon Strategy may no longer be a guaranteed 3-Star attack with the addition of the third air defense in the 2016 update however, it is still a Very Good attack if done correctly).
* If you would like to learn more about using Hog Riders during war or would like to learn about Town Hall 8 & above War Strategies, then be sure to check out the Advanced War Tactics page.
