It is generally agreed that there are 7 layers to the human aura which can fan out as far as a metre or two from the human body. It is the layer closest to the skin which relates to health and is most easily seen.
Human aura's can be sensed by a variety of the senses, including, touch, smell, taste and intuition. By far the most common way to read the human aura is to see it with the sense of vision.
Aura's are most easily seen with peripheral vision, the 40 degree or so at each side of the visual field which can only be seen by one eye rather that the 120 degrees to the front which can be seen by both eyes.
An easy way to learn to see your own aura is to sit in front of a mirror in dimly light conditions. Put a lit candle on a surface behind you so that it cannot be seen through the mirror but creates a glow surrounding you
Stare into your own eyes in the mirror. As you concentrate on your own breathing you will become aware eventually of your own aura illuminated by the candle glow. It may take more than one session before you see it
Once you can perceive your Aura in the candle glow try it without the candle, you may see your aura out of the corner of your eyes
Once you can reliably perceive your own aura you may wish to move on to perceiving the aura's of other people. I actually find this easier if the person who's aura I wish to see is sitting in front of a plain dark background, you may prefer white.
If you defocus slightly and look past the head and shoulders of the person whose aura you wish to see, you will eventually become aware of a faint white or yellow auric glow around the person, with practice you should become able to visualise the colours in people's auric fields.