how to get there from beijing south railway station? and what is the best way to get to china great wall from my hotel?

Subway is the best option. You may walk along Nanheyan Street until you reach East Chang'an Street. Then walk along the street to find Tiananmendong subway Station and take suway line 1 to get off at Xidan subway Station. There you can change subway line 4 to reach Beijing South. And you can take a taxi too. It costs around CNY 30 if there is no traffic jam.

Yes, Yikatong card can be used on bus and subway. Also you can pay taxi charge with it too. Normally, you can buy the card at every subway station.