What you will need to write is from scratch is.... the rest of your life. Linux is based on 40 years of software, has thousands of man years of coding effort. Windows similar, OSX and Andriod are based on Linux with several thousand more years added.
Its ignorant, presumptuous and naive to think you have a hope of getting anything more than but a rudimentary task scheduler going if you start from scratch. It's great that you think you might be the next Linus T, but my guess is you are not. (I will eat my hat if you prove me wrong in 20 years)
Before you start, have a read up on what resources Google needed to turn Linux into Andriod, or Apple used to turn Linux into OSx, then look under the bonnet and see how close they are to the baseline they started with (i.e. How much is the same)......
So to answer you first question - Will starting from scratch be WAY to hard? - Absolutely, in the unlikely event it's not too hard technically, it definitely will take too long.
The next question - what you need to know - A little of Everything, a lot of most, everything about a little.
Last question - where to start - learn to reign in grandiose plans to a goal that is achievable and realistic.