can anyone please say. how to go great wall by bus from Beijing capital airport terminal 2 ?

Could you please specify which section you are going to visit? There are several sections that worth your visit in Beijing. If this is your first time to China, I personally recommend the Badaling section for you.

To get to Badaling, you can first take airport express to Dongzhimen, get off and change to subway line 2 to Jishuitan Station, exit the station from Exit B2 and walk for about 500 meters to Deshengmen, where you may take bus No. 877 to Badaling. The bus ride takes about an hour and the fare is CNY12 per person. The other option is to take subway line 13 upon arrival at Dongzhimen, get off at Huoying Station (Exit G4), and then walk to Huangtudian Railway Station for train S2 to the Great Wall. The train ride takes about 1.5 hours and the fare is CNY7.