When you post your status from the Facebook website (or from many mobile apps), you have the following pull-down menu available to you, from which you can decide exactly who to share your update with:
Facebook remembers your selection, and your next post will default to the same audience. If you regularly change between making public posts and posts only for friends, remember to double check this setting when you post.
If you accidentally post to public, meaning to share with only your friends or a list, this is easy to fix. Simply go to the post on your timeline (in my experience, you cannot do this from mobile apps. You need to be on the full website). Go to the post, and you will find the same pull down menu next to the date that you posted the status or picture: