How to improve your photo
Flash. It's not as easy as it seems. The trick to using flash is (for me at least) to get natural looking lighting. In my experience, the best way to do that is to NEVER use the flash that's built into the camera. If you can, an accessory flash unit, bounced off the ceiling or otherwise modified, will make a dramatic improvement with your flash photography. I use a Sto-Fen or Gary Fong modifier, usually aimed toward the ceiling.
The black t-shirt is a problem, especially when dealing with the kind of lighting you've used. If your camera allows, always shoot in the RAW format. Using software like Adobe's Lightroom or probably even the software that came with your camera (or whatever you used to go to B&W), you can usually adjust the shadow detail so that there's separation between the shirt and the background, without effecting other regions of the photo. JPEG files don't always allow that kind of flexibility, and the results aren't usually as seamless as with RAW.
One other thing to try: Use a slower shutter speed with flash (1/30, 1/15). That allows the ambient light to register and will bring some background detail out of the shadows. Experiment, play around, have fun!