There are several ways of doing this. The first method works under version 2.0 and 3.0, it takes the most work, but looks terrific if done correctly. The second method works under 2.0 and 3.0, and is easier, 3.0's version looks better due to the textures involved. The third method use Lens Flare and works only under 3.0.
You can refer to an easy example, a simple lightbulb. Think 2D for a bit, remember that the visible lightsource described is the type you'd see on film. What's a light look like? Well, there's a central disk of light that seems to be a constant brightness, the bulb itself. As you go further outward a secondary disk of light surrounds the central disk: this is the first halo, it is roughly twice twice the size of the central disk. It fades out rather quicky, it's, essentially, the part of the light that "burns" the film. Go further outward. There's a second halo, it's very dim and eventually fades out. This halo varies in size, but can usually be set to eight times the size of the original disk. This is the halo that is create when the light is dispersed by atmospheric conditions. So with that in mind:
This is a basic "Light w/halo" object. When rendering it be sure to always point the Y axis at the camera. It's easy to do: just Align Y axis to Track to Object Camera. Render that, and see what you get. In this example the smallest disk acts like the bright lightbulb, the second disk acts like the film burn area, and the third disk acts like the atmospheric dispersion area. The object, being 2D, has some limitations.
A primary limitation is that this object only works well in Scanline situations, this is because in Trace Mode the two larger Radial mapped spheres block the smaller disk's light travelling backwards. To work around it create an axis, make it a Lightsource, place it behind the larger disk, group it to the smallest disk. It isn't infallible, however...
Experiment with values. The Color values given above were for a standard high intesity lightsource. Always remember what your lightsource would look like in the real world.
If you're using Imagine 3.0 you can use the Sparkle texture. If you've downloaded Milan's textures (BTW, Thanx, Milan!) there is one itx in there that is useful, but my memory fails me as to the name. I don't have essence, so I don't know what there might be in there. Hope this helps...
You could use the Lens Flare effect that comes with Imagine 3.0, just disable the flaring and have only the halo turned on. Easy.