what bus to take from Qianmen to Deshengmen for the bus No. 919 to the great wall?

I'll be in Beijing on the 4 june 2010 for 5 days with my wife and family. We are staying in a hotel in front of Qianmen. we wanna to see the Badaling Great Wall. Can someone please tell me how are we going to take the subway or what bus to take from Qianmen to Deshengmen bus station for the bus No. 919 to the great wall? thanks.