In Unity you can hand craft your terrain or use a terrain generation tool. This article will offer learning resources for both methods and some terrain tips.
If you wish to handcraft your terrain or even if you're using a terrain generation tool it’s a good idea to have an overview of how Unity’s terrain engine works.
Unity Manual – Terrain: Unity Manual
Videos: Search for Unity3D 5 terrain tutorials. Recommended videos: Wicked Cat: Wicked Cat Studios Check the dates of the videos view the newer ones.
On the Unity Asset store there are several tools to generate your terrain automatically or with various degrees of control.
Gaia - https://www.assetstore.unity3d.com/en/#!/content/42618
World Creator - https://www.assetstore.unity3d.com/en/#!/content/54631
MapMagic World Generator - https://www.assetstore.unity3d.com/en/#!/content/56762
GeNa - https://www.assetstore.unity3d.com/en/#!/content/74407
Landscape Auto Material - https://www.assetstore.unity3d.com/en/#!/content/64132
And if you like to sculpt your terrains out of mesh, here is a popular utility: https://www.assetstore.unity3d.com/en/#!/content/67758
Please check the ratings and read the reviews, also look at when the package was last updated.
Here are some articles on terrain optimization.
3D Ace: https://3d-ace.com/press-room/articles/optimizing-environment-3d-models-in-unity3d
Here are two articles on optimization, please note there are scripting portions that you can ignore unless you are going to be using the space SDK.
Last tip can be found on the Gaia website:
http://www.procedural-worlds.com/gaia/tutorials/mobile-unity-scene-optimisation-with-gaia/