Hi, you did not tell which section of the Great Wall you would like to visit, but I suppose you will visit Badaling Great Wall, the most famous section. So you should first take a flight from Hongkong to Beijing, and in Beijing, go to Deshengmen Gate Station where you will see many 919 buses. You will probably be puzzled on which 919 you should take. Let me tell you, you should take No.919 (快"meaning fast"), on the bus board, it is cleared written "DE SHENG MEN - BA DA LING" and the ticket costs CNY 12. The bus is green. It will take you about one hour to Badaling Great Wall.