That really depends on what you are after. A good point to start is to use a Box Emitter and have it fill the entire scene. Then simply emit particles, but set their velocity to very low values or even zero. For "floatiness", make your particles move using the Turbulence. You should have some slight gravity so they still sink somewhat to the ground. In addition to that, you may want to use the Wind X and animate it with vraying intensities, so your particles have a slight flow from side to side.