Hi!

To create an infinite ground (which is what I supose you are asking) you can use the top button on the toolbar on the left. Right click there, choose ground and then left click it to create it. But this is not exactly a "terrain", this is just the ground. A "terrain" can not be infinite. Terrains can be created on the 4th button from the top in the same toolbar. After created, you can resize it to a very big dimension but it is not infinite. But I strongly recommend that you read the manual...

Regarding your 2nd question, everyone would like to have that answer! Tell me if you find it... LOL
I know for myself that sometimes I make something I find stupid and other people like it and other times I really try to make something good and people don't like it. So, who knows?!...
Just follow your instinct and start creating. You will learn as you go.

Regards,
Artur