All you have to do is play MW2 online on the 5th and/or 6th December. The more players who are on Xbox Live playing = the more money ActiBlizzard will give to an awesome cause.
Also GAME will be donating a £1 from every sale of Modern Warfare 2 to the same charity, as well as selling War Child wrist bands in store towards the end of this week.
You can fine more information on GAME For Good here, and to join the Facebook group supporting this event here's the link.