Introduction to Mendel from Rep Rap on Vimeo.
You've decided you're going to make a RepRap machine? Great! Now what?
The first thing you're going to want to do is decide which RepRap machine you're going to make. It's pretty easy to say, "the most recent version," because that's the Mendel. However, it turns out there are a number of versions of the Mendel to choose from. Sells, Isaac, Prusa, Huxley, these are all the names of different flavors of the Mendel. They all have the same basic shape, but they vary in some subtle ways. The Sells Mendel with Gen3 electronics is pictured on the RepRap home page (the video link) and is featured in the introductory videos, etc. It's named after Ed Sells, the guy talking in the above video. He developed the Mendel and the Sells Mendel is the original. The Sells Mendel has some drawbacks, however, mainly that the complexity of the design makes it more difficult to dial in and get working than other variants.
The hot variant right now is the Prusa (pronounced PROO • shuh) because it has fewer pieces, is easier to build, and is more standardized. It too is named after it's chief developer. I found this video, which compares the two models.
These just give you the basic frame and mechanics of the machine. You will need to add the extruder, electronics and motors after that. There are a number of electronics packages to choose from. Right now the hot stuff is the RAMPS configuration. I understand the Wade Extruder is the thing to get in that department. The motors are just stepper motors of a certain specification, which is described in the plans.
Step one, pick a variant, electronics and extruder. For me, that's Prusa with RAMPS (w/ special attention to getting it with the SD card reader) and Wade Extruder.