Well, of course. Where are you at?
If you are at the point where you can use C to "start" to read from the disk, does this mean you have a boot loader which has now loaded your kernel? Have you made it that far yet?
If you are at the boot loader part, you need to write the boot code and use the firmware (Legacy BIOS or UEFI) and read from the disk that way.
If you have a loaded kernel, you need to write a driver for the desired media, in this case, probably an ATA drive.
Where are you at? Boot loader? Have a look at
https://github.com/fysnet/FYSOS. This is the boot loader code for the book series I might of hinted to before.
If you are at the Kernel stage, there is a post at
viewtopic.php?f=1&t=33152 of someone asking how to read from the disk.
- Ben