There’s a wide number of track plan books available to show you how to design a model railway, additionally just look at any model railway magazine or go to a model railway show for inspiration.
If your looking to design your own model railway its worth while checking out the program Any rail, as its quick and easy to use, for the more advanced you can use templot but this does require some knowledge to work. Or you could quite simply draw it out on a piece of paper!
If this is your first model railway, its recomended to start simple, dont try and do something ambitious like Kings Cross, keep it simple. Remember this is a hobby you should be able to enjoy it.