I'm a former portrait shooter who made a full-time living from shooting studio portraits in the early 1990's, so I will give you a couple of pointers about what makes a good portrait. First off, and this might rankle some amateur feathers here, but ditch the DX camera and buy a FF digital SLR, or at least ADD a FF body to your kit; DX is over twice as small as FX, and the way lenses behave on DX is disadvantageous to you as a portrait shooter both indoors and outdoors. To shoot a 6 foot tall person on FX, with an 85mm lens, you can stand 20 feet away. With a DX camera, you need to be 34 feet way for the same field of view. And at 34 feet, the DOF is hugely increased. Indoors, in small studios, DX means ultra-short focal lengths to get the width you need to encompass groups, and that means HUGE depth of field, and almost no background control. So, look into getting an FX Nikon body. Seriously. It will make your work look more-professional, and make your life much easier.
Second. The key to a good portrait shot besides equipment, is rapport with the subject and confidence-building between you and your subject. Confidence on your part translates to confidence on their part. You need to learn how to look confident, and sound confident, and learn how to simply and clearly tell your subjects how to pose using verbal instructions.
Most volume shooters and fast-paced shooters use pack and head studio strobe systems. Monolights need a boatload of electrical outlets for a six-light shoot,and frankly, most monolights are very one-dimensional,without enough flexibility in terms of heads offered, or the ability to focus or flood the beam spread, have no Fresnel spot accessories, etc. As a new shooter starting out, unless you have a trust fund, stick with Speedotron pack and head systems using mostly 102 light units and 805 or 2400-series power packs bought used. Soooooooooo many people go broke in photography that buying used Speedotron lighting allows you to assemble a complete, entire studio with five 102 light heads and 2- 202VF (Variable Focusing) heads, three power packs, eight light stands, one heavy boom stand, 4 each of 7 and 11.5 inch reflectors, and one-22 inch beauty dish with grid, 3 or 4 barndoor sets, honeycomb grid sets for the 7 and 11.5 inch reflectors--all of this can be bought used for the cost of ONE higher-end Profoto pack and one light head. I'm not kidding on the prices either.
I know other people like other brands of lighting gear, and many will suggest Alien Bees, but I think you'll get more respect,and more done with less hassle by buying Speedotron Black Line equipment on the USED market,and specifically from e-Bay or craigslist.
Lens wise, an FX format camera makes a 70-200 a very useful lens; on DX, the FOV crop makes a 70-200 much less-useful indoors. The 105 DC and 85mm 1.4 AF-D Nikkors are superb portrait lenses; on DX, indoors, they become instantly LESS-useful, by far.
There really is no one,single key to getting good portraits--there are many things you must do correctly, but owning or having access to the right equipment will make things easier than having to continually "fight" your camera,lenses,and lights.