Welcome!!! 
-use a combination between world machine (or a similar tool) and the landscape tool
-take a look at some landscapes from epic games -> learn tab of the launcher (kite demo, landscape demo,…)
-get used to the UE4 material system -> a good material is crucial for a landscape
Some useful links:
basics - https://docs.unrealengine/latest/INT/Engine/Landscape/QuickStart/index.html
all information - https://docs.unrealengine/latest/INT/Engine/Landscape/index.html