It is very very simple work to add share button on your webpage . there are ready made code available for that .
you have to just go to this
url:- https://www.addthis.com/get/sharing?lb=1&flag=register#.T6S7X1JEjIU.
in this url , you will find theĀ code of allĀ share button like facebook,twittrer, google+,gmail ,yahoomail and lots more near about there are 50 to 60 share button on this page .
you have to just copy and paste these buttons code.
You have privilege to add your custom button in that .
Adding “share button” is only matter of copy and paste , nothing else.
Thanks !!!!