Softboxes have two diffusion baffles and can angle the light with more direction, helpful for edgy portraits. Also, if you're working in a tight space, softboxes won't spill light onto backgrounds and walls as much as umbrellas.
Another useful accessory is a simple white/silver reflector, which can be used in the portrait setups as illustrated in this article.
Additional items needed are light stands and backdrops. Simple light stands like the Manfrotto 367B nine-foot stands should work fine. Fill some stuff sacks (find them at a camping store) with sand, and you have sandbags to hold your stands steady.
One background that works for all three lighting setups is a white seamless. White looks great for most subjects, and you can change the color from white to gray if you move your lights and subject away from the background. For a little more money, you can buy a roll of black seamless, or buy a black velvet background you can hang from the wall. I also really like a patterned background like the Lastolite Urban Background. These pop up like a big reflector, so you can lean them against any wall and you're ready to go.
If you're like most photographers, once you start shooting with the basic gear needed, you'll start justifying buying more gear (I'm the worst!). Sure, you'll get softer light with bigger softboxes, and boom arms are really handy, but remember, you can create terrific portraits with a few umbrellas, stands and a couple of lights.
THE LIGHTING SETUPWhat about the lighting setups? Try out any of these lighting techniques with your subject, and you're off and running with your studio business.
The Executive Headshot. The beauty (no pun intended) of executive headshots is that your subjects need professional images to promote themselves in business, meaning they're willing to pay for it. The last thing a young professional wants on his or her business card is a horrible headshot. Some photographers make their entire career around shooting headshots: actors, executives, athletes—they all need them.