That isn't too bad! Actually very good. First I'd start by getting some seamless white paper ( the widest you could find ), that would alleviate the seam you see in the background. From there, if you have or are using a tripod to hold your camera, experiment with the shutter speeds to see if you can brighten up or overexpose the background. The longer you have the shutter open, the brighter the image will be, but you know that already.
You might try using your on camera flash to put some fill in there and see what that looks like. If you're using Canon, just try starting with the ETTL setting on it ( basically your letting the camera figure out the flash output power). If that doesn't look good or looks funky, then go to manual output on the flash and maybe start with 1/8 power and see what that looks like. Oh, forgot to mention to keep you lightbox lights on while doing this too!