Shangdi in Haidian District, right? If so, you are close to Shangdi or Qinghe Station of Subway Line 13. Take the subway to Xizhimen first. Then change to Line 2 (just one stop) to Jishuitan. Get off from Exit A and walk eastward to Deshengmen Gate. Behind the gate, there is the bus station for 919. Take the bus to Badaling Great Wall.