Yesterday I asked where to begin. A good place to begin is with an idea of what you want your terrain to look like, and then start looking for photos to get ideas.
Also, an understanding of why battles are fought where they are fought helps as well. In gaming we tend not to think towards the why just the where - most games I've played over the years have been the equivalent of playground basketball games - sides are picked, troops sorted out to the various players, and to misquote Sherlock Holmes, the game is then afoot. But at no time (even when playing historicals) has a lot of thought been given to the where and why - terrain is assembled as an afterthought a lot of times - a scattering of trees, a road, a river and a few hills and you are ready to play almost anything - from Ogg and Slogg the Orc(k) brothers slugging it out up to and including 40K (yes I have played games of 40K where we had Space Marines running around on hill and dale - because we lacked buildings). And before some history major nazi type points out that a whole lot of battles are the equivalent of pickup basketball games I understand that - but the vast majority of major battles and even minor ones are not the result of just blundering about on random terrain until you meet the enemy.
I understand that the game is the thing - but as gamers we should put at least as much effort into the surface we play on as we do into the minis we play with, right?
So, you should have a plan. My plan oddly enough is a desert themed planet (my orkses are already by and large on or going on desert themed bases). Based very loosely on where I have spent 5 of the last 7 years since 2004, and what I have seen there, along with portions of the American Southwestern Desert.
Why? Because its my little world.