It is very easy thanks to the fact that the BIOS already have many input/output functions built into it for changing screen mode, changing pixel color, writing text to the screen and many other things. However, it does not include support for file systems, that is one of very few things you must incorporate in your OS.
The BIOS loads the first sector on the hard drive and starts executing the code from there, so your OS must be placed with the first instruction in the first sector on the hard drive.
This Wikipedia article should get you started on BIOS interrupts for writing text to the screen, receiving keys from the keyboard and other stuff.
https://en.wikipedia.org/wiki/BIOS_interrupt_call
Even if you plan to use c++ I would strongly recommend reading up on assembly programming, as it is vital to understanding how the hardware works.