To my knowledge, there is, regrettably, no textbook-style source on the theory of definition. However, Definitions and Definability: Philosophical Perspectives (edited by J. H. Fetzer et al, Springer, Synthese Library, 1991) is quite accessible to the novice and freely available from Springer.
As it is for many topics in philosophy, I would recommend you to read about the historical development of the notion as a smooth way to have a grip on it. In this regard, Essays on Definition (edited by J. C. Sager with an introduction by A. Rey, John Benjamins Publishing Company, 2000) and Definition in Greek Philosophy (edited by D. Charles, Oxford University Press, 2010) can be helpful.
N. Belnap's 1993 article "On Rigorous Definitions" (Philosophical Studies 72, pp. 115-146) is informative about recent discussions. There is also R. Robinson's Definition (Clarendon Press, 1950); however, it may remain too outdated.