A really good, in depth, explanation regarding Depth Of Field when using your camera. The post is a few weeks old on the Petapixel blog and is well worth a read if you like photography. The post explains all the basics and some of the more advanced features that will help you take better photographs, even if you are using a smartphone.
The basic camera app that comes with your phone probably won't have depth of field settings that are adjustable as it is usually a point and shoot (with the camera deciding the right settings for the scene). I recommend the use of the Camera+ application that can be found over at campl.us and has been a staple on my iPhone since the app launched. Even better it costs less than your morning latte from Starbucks, even with the add on packs. Lisa Bettany one of the developers has a great instagram account of which most of her photographs are taken with an iPhone using her app.