I’ve provided a few tutorials, guides, and websites which may assist you in environment creation, level design, and more.
There’s a lot of sources out there, but you’d have to start slow and with basic stuff first since there’s a ton of steps in the workflow.
EDIT: You’d typically start with modelling the meshes or models in a program like Blender or Maya, then you’d port that to a texturing program (or texture it in Photoshop, etc.) like Substance Painter; then you’d import that into UE4 and start utilizing it. It’s a fairly interesting process, the ‘WorldofLevelDesign’ site should help you with sceneries.