First you need to understand exposure. If you are using a higher ISO than you would prefer, you need to let more light in with either a longer shutter speed, or a wider aperture. See:

• What is the "exposure triangle"?
• What is the relationship between ISO, aperture, and shutter speed?

The Nikkor AF-S 18-105mm has a maximum aperture of f/3.5 which isn't all that generous, although quite typical for an entry level kit lens.

So to start out with, are you shooting at the maximum aperture that your lens allows? Try aperture priority mode and f/3.5(you will have to zoom out to do this as your lens has a variable maximum aperture depending on how much you zoom in). More on variable maximum aperture lenses:

• Why do zoom lenses and compact cameras have varied maximum aperture across the zoom range?
• What are the advantages of a lens having a fixed maximum aperture?
• Why does the maximum aperture change with focal length?

Second, are you shooting at a slow enough shutter speed for your conditions? Indoors with a lens like that and depending on your subject, you may want to shoot at 1/125th second, maybe slower or maybe faster but you could start approximately in that range. See:

• What is the shutter speed needed to stop motion?
• Focus problem vs. motion blur vs. camera shake - how to tell the difference?

Finally, if you just can't get enough light with the equipment you have, in the conditions you want to shoot - you may want to invest in equipment. A decent off camera flash is where I would start, but you could also get a fast prime lens for not much money. See:

• What does "Strobist" in Photography mean? and
• Prime lens or flash: which upgrade will most improve baby photos?