Look up the BVE tutorials by dennislance , they can be found online using wayback machine.
Everything is created using text code and text editing, there isnt a graphical interface such as you might find in some modern games.
It is a very slow and painstaking process, particularly if you intend to create you own scenery objects.
My advice? Forget 'all of London overground'. Build yourself a short, imaginary route of around 5 miles or so, if you enjoy that then aim bigger. hth