Here are my recommendations (as a non-expert) of reasonable-length sources introducing "natural law":
Thomas Aquinas, Treatise on Law (Regnery Gateway 1996) (Hey, I said "reasonable-length," not "easy!").
J. Budziszewski, Natural Law for Lawyers (Blackstone Legal Fellowship 2006); if this is unavailable, try the slightly longer Written on the Heart: The Case for Natural Law (IVP 1997).
William Blackstone, Of the Nature of Laws in General, from his Commentaries on the Laws of England, Introduction, section 2. The link here is to the full text of the essay from a complete online edition of the Commentaries, nicely indexed online from the LONANG library of historic reference works.
This will get anybody, at any level, started.