He believed that the American colonies had the right to be free from the British and so his idea of freedom helped the Declaration of Independence be formed.
He specifically attacked the idea of a monarchy and hereditary aristocracy. He goes on to say it is absurd than an island governs a continent in his most prominent work, Common Sense.
He also was a strong supporter of freethinking. He is controversially known for attacking institutionalized religion, and advocating deism. All of this comes mostly from his 3 part book -- The Age of Reason (1794- 1807, released in separate parts).
Another important belief, shown in Common Sense and Rights of Man (supporting the French Revolution), is that a government should always protect the citizenry and their rights and interests. He advocated revolution and independence if necessary.