buses going up and down beijing - badaling great wall vice versa/train ticket counter

I notice there 2 buses that are bus no 877 and 919 going to and fro beijing Badaling Great Wall. Is it correct?
can we buy bus ticket just going up to Badaling only - and How much one way cost ?( adult /children ).
coming back by rail
which one- The S or the Y588 ?' and where can we buy the coming back rail ticket - is it at the Badaling station counter ? /how much does it cost ?
or there is website to buy Online rail ticket Badaling - Beijing North Station