Below is a worked example (for the CA65 assembler) on how to load a file called "MAPS.BIN" into a memory location (map_location) in assembly. As you can see, you first have to call the SETLFS and SETNAM functions, after which you can call the LOAD function. See this page for some documentation on these Commodore 64 API calls.
IMPORTANT: if you are using the emulator, the file name on your file system should be in UPPER CASE. The file name in the code should be in lower case.
B.t.w. Can anybody tell me how to properly format code on this forum?
Edit: sorry, didn't read your post properly, I assumed you were talking about using assembly, not C.
.macro ram_load filename_start, filename_end, ram_address ; overwrites A, X, Y
SETLFS = $FFBA
SETNAM = $FFBD
LOAD = $FFD5
filename_length = filename_end - filename_start
ram_load map_filename, map_filename_end, map_location