This i likely a bigger question than you believe it to be.
You want to take a look at a few tutorials on YouTube. Here’s a good enough place to start as any:
Then when you’ve sculpted your terrain take a look here for applying textures to it:
Youtube is your friend here, Iv managed to make a few pretty complex things in UE4 from nothing more than what i have learned there.
Tesla makes some good tutorials and then when your feeling more confident take a look at UnrealGaimeDev (Yes is pelt that correctly) 
If you question is specific than that, we’re going to need to see what you mean so its a good idea to post images along with questions otherwise it can be very hard to understand what a problem is exactly.
Good luck.