How to pick a flash for a Nikon dslr?

I have always enjoyed photography as a hobby and have even worked in a studio. (a long time ago) I want to take my hobby up a notch and while the technical side of photography makes perfect sense to me I'm struggling with the lighting portion. I feel the best way to get better with it is to go out and practice but I'm not sure where to start with lighting equipment. I will be using a Nikon D7100 and would like to start with standard portrait work. As of now I will not be using it for professional work but the ability to move into pro work with my new equipment would be a plus. Any recommendations? Any thoughts on speedlights in particular?

Get the best Nikon Speedlight you can afford. For on camera flashes, I really like using the camera's branded unit. They are not much more money and are built to work as a system. Anyone else reverse engineers the TTL controls.