Nikon D5100 best quality?

I just got the Nikon D5100 and i'm interested in which specifications should i apply to have the best quality in photos and videos… Especially in videos.
P.S. What are does NEF files, i saw that they are min 15 mb and more large?

The best video is 1920 x 1080p -- Full HD video

Best still is to shoot in RAW and process them in Lightroom 3 or 4

They can be exported as TIFF files if you want to produce huge prints. Export them as high resolution JPEG files for smaller enlargements.

NEF are raw files. Unprocessed sensor data that hasn't been turned into jpeg. That file format is the most versatile in terms of manipulating the image, but after manipulation you must convert it into a standard file like jpeg, tiff, etc.

As for for photos it really depends on the lighting conditions and what you are after. Focused subject blurred background or everything focused, moving things are blurred but stationary things aren't.

Basically what you are asking can't be answered in one response. If I were you I would just play around with it then review the photos and see which I like. Then try to reproduce that same result. Also it doesn't hurt to read up on photography such as sports, landscape, portraits, night time.