Hello ! Can anyone tell me the route from PEK(Airport) to Great Wall ?

Since you just have 18 to 19 hours, I suggest you visit Badaling section. You can take airport express line to Dongzhimen. Then change subway line to Jishuitan Station, get out from Exit A and walk to Deshengmen. There you can take bus no. 877 to the wall.

Thank you Billy ! And Exactly how much time will this all take ? And These places you told, If i say them in English will the people be able to understand me?

Airport - Dongzhimen: around 40 minutes
Dongzhimen - Jishuitan: 10 to 15 minutes
Jishuitan - Deshengmen: around 10 minutes
Deshengmen - the WALL: around 1 hour and 20 minutes

Actually, you needn't say these addresses in English. Just show them to other people if you need help. They will understand what you mean.

Thank you very much. You have been very helpful.