Can i change who sees my posts on facebook?

[alidan1984]

I belong to a group called sellers club and am wanting to buy my sister something but I found out my friends see everything I write on that wall. How do I change it so she won't see what im writing?
Is the button with the pic of a little person with the plus sign next to it? If so I never check mark anyones name and everyone still sees it.

[keyvan]

When ur typing there should be in the corner a button so u. Can choose who sees it