I'm assuming you are trying to capture a close up portrait with bokeh. You would want to use a lens with a short focal length and a wide apeture to get this effect. A lens with an f stop of less than 2 is probably best. For example, if you are using a Canon EF camera you could use a lens like this Canon EF 50mm f/1.8 lens.. You also want to be relatively close to your subject with a distant background to being the background more out of focus.