Let’s first understand what karma is.
Karma simply means the cause and effect. And karma happens in every area of our lives. Be it good or bad karma, karma exists in every aspect of our lives, and in the nature. 

Karma exists in all things. It exists in the body, the mind and our spirit.
Whenever there is an input of an action, a circle of karma starts. 

Your Life’s Karma Is Made Up
By 3 Main Areas Simultaneously:
The Nature Function of Body, Mind and Spirit.

The Body Karma, Physical Area:
For example, you eat a piece of meat, that piece of meat will become you eventually. The food that you ate simply makes your flesh and blood. When the body dies, it decomposes and turned into essential nutrients or soil for other living organism to feed on. This is a circle of the body karma.

The Mind Karma, Thoughts :
The mind creates chains of circles of karma within itself. The ego state of the mind often constitutes as the root of accumulative negative karma. When you have a resentment or anger, those negative thoughts become accumulative and escalates within the mind, it is intensified every time those thoughts stay un-transcended. 
Like the body, the mind also has a capacity, a circle of life and death of a state of thoughts. 
Simply, your thoughts are the food for the mind.

The Spirit Karma, Spiritual Area:
Your reality and life experiences are essentially created and manifested by the spiritual area.

Reincarnation is the karmic circle that roots from the spirit.
All actions and thoughts get imprinted to the spiritual area and the spiritual area manifest into your reality.

To change your reality, you have to master the way to change all attachments within your spiritual space.

Our minds have the heaviest impact on our spiritual area. Master the mind in the first stage will be essential to change and control your destiny.