What camera should i purchase to starting to take more professional pictures with?

I've always used digital cameras. But now i want to look into getting a more professional camera.what's a good nikon slr camera to start out with? My price range is under $600.

The only Nikon DSLR under $600 is the D3000

if you up your budget to $700, you can get a better camera with more features, video, a better sensor, and more, the D3100