I'd second reading Chip's beginners guide. Only takes a couple of minutes and it's time well spent.
You might also begin thinking of some of the design criteria that you haven't yet mentioned but is crucial to the design process.
Do you want point to point, continuous run (like a circle), point to loop, loop to loop, switchback.........in other words, what style of running interests you?
How about duckunders? Is a duckunder OK, or do you want a walk-in design? What about a swing gate or a drop leaf?
There are a ton of things that have to be decided before pen ever meets paper (or fingers meet keyboard & mouse as the case may be).
A model railroad can consume a lot of time and money. Do your homework and do it right the first time.
Read, ask questions, dream, ask more questions, read more.....then you'll have a good idea of what you're after and how to go about doing it.
Seriously though, give Chip's beginners guide a read. You won't be sorry.