Taking subway is the cheapest and fastest one. At Tiananmen Square you could take the subway Line 1 and then transfer to Line 5 at Dongdan Station. Get off at Huixinxijie Nankou and then take Line 10 here, transfer to Line 8 (the Olympic Line) at Beitucheng. Finally get off at Olympic Center where the Bird Nest is located. It takes only RMB2 and 80 minutes.