What are the major beliefs of Scientology?

Scientology was established as a secular philosophy in 1952 by science-fiction author L. Ron Hubbard, then recharacterised by him in 1953 as an "applied religious philosophy."
Scientology beliefs are structured in a series of levels, with new initiates working their way up by steps to the more advanced strata of esoteric knowledge (ALL levels must be purchased). This is described as the passage up "the bridge to total freedom," or simply "the bridge." The more advanced teachings are kept strictly confidential.
The central beliefs of Scientology are that:

• a person is an immortal spiritual being (termed a thetan) who possesses a mind and a body, accompanied by a lesser "genetic entity";
• the thetan has lived through many past lives, stored memories of which can cause problems in the present day;
• a person is basically good, but is "aberrated" by the memories of past traumas.

Alternate Answer:
The major beliefs of Scientology are stated in 'The Creed of The Church of Scientology'.
　
We of the Church believe:

That all men of whatever race, color or creed were created with equal rights;

That all men have inalienable rights to their own religious practices and their performance;

That all men have inalienable rights to their own lives;

That all men have inalienable rights to their sanity;

That all men have inalienable rights to their own defense;

That all men have inalienable rights to conceive, choose, assist or support their own organizations, churches and governments;

That all men have inalienable rights to think freely, to talk freely, to write freely their own opinions and to counter or utter or write upon the opinions of others;
That all men have inalienable rights to the creation of their own kind;

That the souls of men have the rights of men;
That the study of the mind and the healing of mentally caused ills should not be alienated from religion or condoned in nonreligious fields;

And that no agency less than God has the power to suspend or set aside these rights, overtly or covertly.

And we of the Church believe:

That man is basically good;

That he is seeking to survive;

That his survival depends upon himself and upon his fellows and his attainment of brotherhood with the universe.

And we of the Church believe that the laws of God forbid man:

To destroy his own kind;

To destroy the sanity of another;

To destroy or enslave another's soul;

To destroy or reduce the survival of one's companions or one's group.

And we of the Church believe that the spirit can be saved and that the spirit alone may save or heal the body.
　
The Creed of the Church of Scientology was written by L. Ron Hubbard shortly after the Church was formed in Los Angeles on February 18, 1954. After he issued this creed from his office in Phoenix, Arizona, the Church of Scientology adopted it as official because it succinctly states what Scientologists believe.

The best way to understand what the beliefs are, is to personally read the basic books of the subject. These books, called 'The Basics', are available to anyone in nearly any public library for free. Most Scientologists have or plan to acquire the complete set of these books for themselves or their family to have as their own, so they can understand the basic beliefs as well as possible.

Scientologists believe the best way to read these books is read them chronologically, in the exact order the author wrote them so one can see the development of the ideas and beliefs for themselves one at a time at one's own speed. There are lectures available for each of the basic books that provide a further and much greater understanding of the principles in each book. You get to hear the author in person, one on one.

These books describe the nature of an individual person from the viewpoint of the author. They include his personal researches and conclusions, and they recognize the contributions of others to the subject, and what was not contributed, as well. Descriptions are provided in great detail for anyone to read or listen to.

These books and lectures have been accepted by Scientologists as a statement of their beliefs and that they are available for all to read and have, only if they wish.