Welcome to the forum, and the wonderful world of Lionel trains!
The answer to your question is entirely up to you! Most hobbyists start small and expand slowly, ever changing their layout. Others plan their 'dream' layout and build it once.
It all depends on how much space you can devote to your layout, how simple or intricate you want it, if you plan on adding more trains and wish to run 2 or more trains on different loops, and lastly, how much funds you can devote to your hobby.

Here are some layout suggestions to start you on your way, complete with scenery suggestions. Have fun!
 http://www.thortrains.net/marx/47fastrack1.html

Larry