How can I make my old posts on Google+ visible only for those people I have added in my circle?
There isn't a way to change the visibility of existing posts. Once they've been posted they're "out there" and visible to everyone you shared them with in the first place. The only thing you could do is delete it and then repost to the correct circles/people.
For new posts click on the
[x] in the "Public" green button and then click on
+ Add circles or people to share with
Then just pick those people or circles you want.