Analogy of operating System

I want to to develop operating system in ‘C’ as a final project of my university I wan to know the analogy of o/s if any one help me about it how can I write the routine?
from where I ve to start? i need complete consulting about it that how any o/s like windows or simply dos working or if there is any inteface b/w custom o/s to vendor os commands .
I just want to make a program that act on my command rather then dos command like for dir if i say ls (pls not mentione it is unix command) and it is able to boot computer like any o/s