I am currently preparing for the Talmud class that I will be giving this coming school year. I always like to start with a general introduction to Torah Sh'ba'al Peh. I believe the best introduction is the Rambam's introduction to the Mishna. Though the introduction to the Mishne Torah is also an excellent introduction to Torah Sh'ba'al Peh it is tailored to act as an introduction to Rambam's particular presentation of Torah Sh'ba'al Peh. If one's task is to learn/teach the other texts of Torah Sh'ba'al Peh then it would seem that the introduction to the Mishna should be one's primary choice.
My method will be to go through the text and paraphrase or translate each mini-unit/paragraph and provide whatever explanations are necessary to understand what the Rambam is doing. The Hebrew text I will be using is from the "daat.ac.il" web site (I will supplement the translation given there with R' Sheilat and R' Qafich's translations). For now, I will be skipping over the Rambam's introductory "song". Skipping over it is not meant to indicate that it is not important just that I need to think about it some more.
The Rambam begins by stating the most basic idea one must have about Torah Sh'ba'al Peh, the Oral Law, before study can begin. However, before discussing it, let me just clarify what the Rambam is not doing:
1) He is not telling us the most basic idea that the Torah (and reason) demands we have. The Rambam presents the most basic ideas we must have in his Hakdama to Chelek and Hilchot Y'sodei HaTorah, The Laws of the Foundations of the Torah (really, all of Sefer HaMada, The Book of the Knowledge). To begin studying the Torah one must already possess the y'sodot, the foundations (after all, you are only allowed to teach Torah to a Yisrael who is a Talmid Hagun - we will assume for now that my students meet those criteria).
2) Additionally, the Rambam is not telling us the most basic ideas we need to study Torah Sh'bichtav, the Written Law.
He is, however, telling us the most basic idea of Torah Sh'ba'al Peh: what it is.
דע, כי כל מצווה שנתן הקב"ה למשה רבנו ע"ה, נתנה לו בפירושה: היה אומר לו המצווה, ואחר כך אומר לו פירושה ועניינה, וכל מה שהוא כולל ספר התורה.
All the mitzvot that God gave to Moshe Rabbeinu, A"H, were given with peirush, explanation. God would tell Moshe the mitzvah and afterwards would tell him its peirush, explanation.
As Rav Sheilat writes in his commentary on the Hakdama to the Mishna (my paraphrase/translation): "The Torah Sh'ba'al Peh transforms the mitzvah from a vague/intangible (Artila'i) idea to a practicable teaching/instruction." This is why the Torah Sh'ba'al Peh is properly called Mitzvah, command in the Hakdama to the Mishne Torah:
א כל המצוות שניתנו לו למשה בסיניי--בפירושן ניתנו, שנאמר "ואתנה לך את לוחות האבן, והתורה והמצוה" (שמות כד,יב): "תורה", זו תורה שבכתב; ו"מצוה", זה פירושה. וציוונו לעשות התורה, על פי המצוה. ומצוה זו, היא הנקראת תורה שבעל פה.
All the mitzvot that were given to Moshe on Sinai -- they were given with their peirush, explanation. As it says, "And I will give you the stone tablets, the Torah and the Mitzvah. "Torah" - this is Torah Sh'bichtav. "Mitzvah" - this is peirushah, its explanation. We are commanded to do the Torah in accordance with the Mitzvah. This "Mitzvah" is called Torah Sh'ba'al Peh.
In other words, only the peirush which facilitates "doing" is called Torah Sh'ba'al Peh - "This "Mitzvah" is called Torah Sh'ba'al Peh" - and no other. Without the "do" it is just an "explanation".
This is not an ad. for Mountain Dew/Do but it's a good mnemonic nonetheless.