Pretty good for your first attempt. These are just a couple thoughts...
One of the things I notice right off the bat is composition. There's quite a bit of empty space at the top. Another is the lighting. With the lighting position and her proximity to the background, you can't expose her properly without overexposing the background. Another thing is there's quite a shadow falling across her face. You could move the light a bit down and to the right, and/or use a reflector. The reflector I use is a piece of cardboard covered with aluminum foil. It's really amazing how much light it reflects back. Also moving the light closer to her makes the shadows a bit softer, as this makes it a larger light source.
You're gonna have fun with off camera flash. What are you using to trigger the SB-600?