Things to consider about your picture:
- The light comes from above and very easily in front of the subject. I bet if you drew a line straight up from his mouth you would hit the light.
- Seems to be a single light source. The shadows don't seem to be filled in, which indicates that a fill flash or reflector was used
- Shadows are defined but still have blurry edges, indicating that a small light source was used, but not necessarily a point source. Maybe a little lightbox was used
- The light almost seems to overexpose the edge of the head and falls off at the knees (assuming the drop is in the camera and not in the mail). I would imagine that the light source is about a meter above the subject.
I would suggest that you shot similarly to this one. If your lamp base is not big enough, you can creatively attach the lamp to the ceiling. The light should be directed straight down and positioned directly in front of you.
You don't mention what camera and lens you have, so I'm assuming a new DSLR and a kit lens (18-55mm). Switch your camera to manual mode and choose f / 5.6 for the aperture (the lenses sharpen a few steps down from the maximum aperture). You have to play with ISO and shutter speed to see what works …
30W is unfortunately not that bright. On the sales page for your lamps you can see how they light a sock from a foot or so away. This is what these lights are designed for: illuminating something small and fairly close … small objects, headshot portraits, etc.
Place an object approximately where your shoulders would be in the frame, read a reading through the camera, and set the shutter speed after setting ISO to 3200. For example, if a speed of 1/500 is good – then we know that we can reduce your ISO. The shutter speed should be in the range of 1/15 to 1/60 or faster, using the lowest ISO you can get. Note that you want to use your camera's Mirror Lock Up function in this area.
Because of the low light intensity, you may have to compromise by using a wider aperture at the expense of sharpness. I won't know until you measure!
I would remove the lightbox for this shot and see how it turns out. If your shadows are more defined than your goal, you need to dim the light (use the softbox). The limitation here is that your softbox can do the lighting also soft … this would mean that you need a smaller box or want to try some homemade light softening solutions.
Due to the low light output, it may not be possible to adequately illuminate the head, shoulders, trunk and legs. If it gets worse, you can always lower the light and make separate exposures for the head, trunk and legs and then fuse them together. It would be nice to do everything at once, of course, but if you lack the equipment, give it up again in the post ;-).
Should I somehow isolate myself as I have white walls so that the light is not reflected?
Yes. Your studio should be dark enough so that the camera only records the light that you want it to be shot from the direction it should come from. Most studios use dark / black painted walls, ceilings and floors for the very reason you describe: white walls reflect the light. You can avoid this by using a very large and open space or by wrapping yourself with dark fabric (think of a human light box).