How can I learn The Three Fundamental Principles?

If you are referring to the Usul (foundations) that pertain to the foundations of religion, and about which Sheikh Muhammad ibn Abd al-Wahhab authored his book Al-Usul al-Thalathah wa Adillatuha (The Three Foundations and Their Proofs), then these foundations are: a person's knowledge of their Lord, the Exalted; their knowledge of their religion; and their knowledge of their Prophet Muhammad, may Allah's peace and blessings be upon him. It is important to be diligent in seeking knowledge from a Sheikh, so that he may clarify what is problematic and explain what is obscure.