To load data from floppy disk or hard disk interrupt 13 (hexadecimal) is used.
Using the interrupt with floppy disk is more complicated (function AH=2 - that can also be used with hard disks - uses C/H/S-Addressing of sectors) than using hard disks (function AH=0x42 - which is not present in very old BIOSes (before the year 2000) - can only be used together with hard disks and not with floppy disks).
To run C code you need a C compiler generating 16-bit code (maybe the Watcom C compiler is still able to do this) or you have to switch to 32-bit protected mode.
The WIKI at "osdev.org" is a good resource for OS programming where you'll find a lot of information.