writing my own OS.....where to start from??

Hi,
IMHO, a man who want to start (ehmmm, sorry 4 my english) writing his own OS need basic theory about scheduler, processes and threads , memory management, and so on...but maybe you already know this things.
Maybe you want to start writing an OS based on microkernel like L4 (very nice IMO ) ....
I am not an expert, i'm only a student but as you already know, writing an OS is not only a boot sector.
After that, you will know how, what and where to do something.
IMO.

PS.
Oh my god, this is my first post!