Tim and Vince....
I have been Model Railroading since 1959, as a Six Year Old...Beginning Quitting and then beginning all over again.
===> My first word of advice....
This too will change.
Getting it right the first time?
Good Luck with that, cause I don't think it's gonna happen.
What I would suggest...
Let us know what Locomotives you intend to use...and how long a Train would you like to run on it ?
How much space do you have to work with?
Here's the thing yet I don't mean to discourage you...but the trick is simply to SURVIVE your first layout.