James, Spencer Haygood has given you all of the right introductory tools, which should keep you busy for a while. I will try to summarize these methods for you.
External evidence is that brand of evidence that is real, physical, and tangible.
Now onto the internal evidence. Basically, transcriptional probability is the evidence derived from studying and determining what the scribe was most likely to have written in a given text. Intrinsic probability is the evidence derived from studying and determining what the biblical author was most likely to have said. Now onto the praxes of textual criticism. My suggestion is that you read authors of each persuasion.