My advice will run counter to some given here.
First, I would keep it flat in spite of very good observations that the world is not (enough of it is - you are modeling only a small part of the world, so do a flat portion for now).
Second, treat this current layout as only short-term and something you want to keep inexpensive and easy to take up and throw away. I guarantee you - absolutely positively guarrantee - that within a short time :
-you will learn a lot quickly, so much so that your skills a year or two from now will be much beyond what they are now to the point that you will not be satisfied with work you did while learning, and you will want to re-do it.
Keep in mind that famous saying about experience: "It's easy to do the second time you do it."
So, I would keep it flat this time, simple or at least not too complicated, and at least for now, inexpensive. Experiment with things that interest you, including making a hill or revine some other non-flat portion. Just try things. In this part of the hobby, mistakes are truly learning experiences and often the only way you develop good skills: you can read and watch others, but . . . .
I would make sure anything expensive, like a good building or accessory, is easily removable to move to the next layout.
Trying materials and stuff and techniques from various sources. I use almost exclusively Woodland Scenics, because I've grown to like working with it. You might be different, but try them and others, too.
View this layout as a classroom to practice various methods and see what you like, and assume that, like nearly everyone on this forum, this first layout will not be the last.
For what it is worth you can see a tour of my layout posted yesterday on the O-27 forum. If you see something that intrigues you, and have questions about anything, e-mail me (e-mail is in my profile).