Just go for "Pretty Big". That will be all you need until you know what the rest are for. I find that R5RS is good, but it does lack the extensions that PLT has added to DrScheme.
edit: I just checked and I guess that both "Pretty Big" and "R5RS" are considered "legacy" in DrScheme 4 and the "Module" language is favored instead. Just make sure that all the files you use with the Module language start with
#lang scheme
Module is a way to specify the language used in the source file rather than globally by the DrScheme interpreter. This means that you can use different languages for different parts of your program by breaking it up into files and indicating in each file the language you're using. If you're just starting out, all you need to worry about is just keeping #lang scheme at the top of all the files you use.
A small note - this declaration is not official Scheme, and needs to be removed if you attempt to use the files in another Scheme interpreter.