There are many options for running geofencing campaigns.  You will need to work with an agency (like Click Laboratory) that can set these up for you, or you will need to work with one of the many platforms available for running geofencing ads yourself.
Setting up the most basic campaign is not hard, but there are lots of little "gotchas"  on really making them work well.  All you really need is of course a budget to use, entering information about your target audience's interests and target devices, uploading the banners or videos you want to use, the landing page, then using their mapping tool to identify where you want the ads to show.