Green Line subway (#8) is best, getting off at "Olympic Sports Ctr". Green line intersects directly with Purple Line (#5), Light Blue Line (#10), Dark Blue Line (#2), Yellow Line (#13) and Changping (light lilac line but... More
Green Line subway (#8) is best, getting off at "Olympic Sports Ctr". Green line intersects directly with Purple Line (#5), Light Blue Line (#10), Dark Blue Line (#2), Yellow Line (#13) and Changping (light lilac line but no # yet). All other subway lines require 2 transfers. So where you board depends on where you are starting from, but you want to end up on the Green Line (#8). There are color coded route maps in all stations and also most subway cars. Also each subway car has an electronic sign that notes all stops on its route, and highlights the current one. Signs are in both Chinese and English, as are oral announcements. You can find a great subway system map by googling "Beijing Subway System", but I can't reproduce the link here.
Have fun!