you've come to the right place yes that is a great space to have.
as to answer the q, where to start depends really on your railway interests, eg location, period, operations, etc, and develop a plan around that--eg do you want to watch trains going past, a relaxing thing to do, then you'll need a roundy-roundy type set up, if you enjoy shunting and moving wagons around then a yard needs to go in too etc etc.
plus there's the budget, and time, how much have you got for each? They are the two biggest limits to model railway plans in my experience!
good luck with the build!
cheers,
Keith