How to take good portrait photos using Nikon D3000 without flash?

I'm using Nikon D3000 Camera but don't have Flash gun. I can use Tripod but I need to know the best settings using Manual (M), Aperture(A), Program(P) mode.

Last time I tried to have portraits using built in Flash but I was failed to fade the light of model's face. Although its Dark inside and no lights are available, but using the Built in flash there must be some good settings so that I can get good quality portraits.

There's so much light reflections on the face and looks so bad!

Please suggest some professional ways to controls the flash lights and settings.

If the light from the flash is too harsh then use a thin tissue or something to diffuse it a bit.

If the model is getting reflections then use some make-up on her face.

Don't use Aperture mode. If it's dark you'll end up with an exposure of a few seconds.

I'd use Manual, 1/60s, or 1/125s, and select the aperture to suit.

You'll have to experiment a bit because the flash exposure will depend on the lens you're using and how far you are from the model. The flash itself will have a Guide number that tells you the distance it can be used over at different apertures.

Slow shutter speed and f8 plus. ( f8 minus and faster shutter speed if you want the backgorund out of focus.) Use a reflector if possible to lighten shadow areas.
The problem with dslr's if they are not full frame is that your depth of field is not as good as that of a full frame sensor dslr but by using a slower shutter speed and a tripod that can be overcome to some extent.

Lower the power

Tips from other users are valuable, however there's an alternative where you can try to use a soft filter to create dreamy effect for your subject.

For sample you can have a look at the link below;
http://www.dslrplaza.com//21-soft-diffuser/54-58mm-citiwide-soft-filters.html

Try using a portrait lens and shoot with wide open aperture?

Shoot outdoors… Or in a room with the drapes open.

Yes, you need to have the right settings, but a good lens, is an important factor too! Of course it's you who is taking the picture and all that, but having always a 50mm in your camera bag is good. Because it has the aperture of 1.8 or 1.4 which gives you nice shallow depth of field, or bokeh.
It's good for portrait, and one of the cheaper lenses! Or you can go with the 35mm 1.8 too.

Hope I helped.
For the aperture, the best would be if you choose the smallest number of your aperture as possible, for example, if you have a standard kit lens, 18-55mm 3.5-5.6 you will choose the aperture of 3.5, so your camera needs to be in M - mode (manual)