How can I make the person in my photographs have soft and plain skin without any blemishes using my nikon dslr??
If you wanted to get the soft effect using the camera only, then probably the best option is to use a filter. The good thing about such filters is that you get the effect instantly on your photo. But unfortunately, the bad part of it is that you don't get a perfect effect same as when it is edited through a software. besides that, you cannot revert back to the clean/original photo.
As an alternative, I suggest editing the photo after shooting so that you are able to adjust the effect with the amount that makes you satisfied.
Photoshop has a built in tool: Click Filter, then go to Blur and choose Smart Blur
Start with the lighting. Blemishes and spot show up more in the transition from light to dark esp if that transition is harsh. Using soft lighting such as softboxes would be ideal but it sounds as though you do not have access to studio lighting so window light through netting would be good. Having light from both sides would reduce help to so having a cheap white reflector or even large piece of white card would a good way to reflect light from the window back on the shadow side of the face.
Next use a wide aperture such as f/2.8 or wider. Use a single point of focus on your camera and focus on the eye nearest to you. This will ensure the eye is sharp but skin not on the same focal plane will be smoothed/blurred.
Finally do not use a macro lens it will bring out too much detail. Use a regular lens. If not being used hi-res you can help further by taking the photo from further away. If you are close up more fine detail is visible but form further away and then cropping you can reduce detail in the skin.
Obviously it helps if the model has good skin to start with.
Using filters can soften the image but they will soften everything - if you want to keep the eyes crisp and sharp then the above should help.
Using software to smooth is still the best option as you can then pick what blemishes to remove and keep the detail in the parts you want.