Here I have corrected any grammatical errors so that it makes complete sense, but I have not changed the style/how well written it is. I hope you find it useful. 
I am applying for a Business Managment degree.
I need some help with my Motivation letter.
Ever since I was a child, I have always dreamed of being a manager. When I was about 6 years old my friends and I were playing different games. The most significant role I played was that of a business manager – I had a store, a restaurant or a factory. This has been my dream field of work ever since.
My greatest motivation is that I want to be more independent in life and work. It is important for me to get the satisfaction of knowing I have succeeded by myself. I have always made my own decisions and I have also learned to take responsibility for them. For example, I decided where to study and what, without the endorsement of my father.
(Endorsement is not the correct word to use here, and I would use "'my father's" rather than "of my father").
I have to mention that my father is involved with business management. Seeing and sometimes living the life of luxury that he lives, and how much fun he has doing what he does, has helped motivate me to go to college so I can work up to what he has. That is a big motivation factor for me, seeing what I could have, and then wanting to have it myself.
My future plan is to open my own business in the field of Human Resource Management and Consulting. This is why I have to gain knowledge and experience in a university background.
With all the uncertainty and political, economic and technological changes in the world, I believe that management must have a solid understanding, supported by a strong academic foundation, of how to guide companies through these challenging times.
When I started my studies at the National High School of Mathematics and Natural Sciences, Sofia, I was determined to get excellent grades and to apply to a University. My studies were concentrated on Biology and Chemistry, but also on Mathematics and English.
(I think you should change the previous sentence to something more like: "My main fields of study were Biology and Chemistry, but I also studied Mathematics and English"). This gave me a broader understanding of the world and was the foundation that I needed to be an intelligent and self-confident young woman. I am able to work alone or as a part of a team, and I like working with people. I am flexible and open-minded, and if it is needed, I enjoy organizing different tasks. I am able to work hard, and I love new challenges. I am also eager to learn new things.
I think that with a college degree, my possibilities would be endless, the starting salaries would be higher, I would get more recognition from companies, better quality jobs, and the satisfaction of succeeding in life.