Promoting a recent post to your fans and/or fans of friends is the simplest and most effective method of Facebook advertising, since it appears in the actual Facebook news feed.

Choose a post within the last three days that has done well (engagement rate of 1% or more), mouse over the post and click on the pencil icon that appears in the upper right. Now click "Promote" and select the audience and dollar amount.  

- Here is a good overview of running the three different types of ads.
- In choosing what post to promote, use the STIR methodand here.
- For an in depth article on running Facebook ads, check out this blog post from KissMetrics.