Do you, in fact, have a C compiler for the PIC18? Something like Microchip's own C18 compiler, or the CCS Compiler, or some such? Because otherwise, you should be using assembly (the opcodes are in the back of any of the PIC18 datasheets--the full sized several hundred page ones, not the 20 page 'brief introduction' documents). If you were using an ATMEL, there would be the GCC compiler instead (there is no official GCC port to PICs because of underlying design constraints).
From a pedagogical (i.e. educational) point of view, assembly language is good because it gives you a deeper understanding of what happens at the hardware level. From a functional point of view, it's probably easier to bang something out and debug C code.
Note that, if you are a student, you can get a student evaluation version of the C18 compiler (from Microchip) which, when it expires after a few months, is still functional, if not as optimized as the full version (this was true as of a few years ago, at any rate):
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&dDocName=en010014&part=SW006011
EDIT: There are other threads which mention PIC-derived development boards which are easier to work with, and offer, for instance BASIC support. Perhaps someone will mention one of those later in this thread, or point out one of the preexisting ones?
