Next to the 'post' button you will see the setting for your audience. Just select 'friends'. Now, if you change the audience for a particular post, you have to remember to reset it afterwards. Facebook doesn't retain a default setting.
Why are all my own posts to my timeline *all* automatically set to "Custom"? (for who can see it)
In my main privacy settings, I have it set to "Friends" under "Who can see my stuff".
How do I fix this? Thanks!