Welcome to the forum. I hope you will get to grips with TG, as any start is difficult. If you have any specific questions, do ask them here.
And yes, TG can import terrains and objects (best are obj) made elsewhere. Imported obj's can be altered and completed with additional shaders within the object (+), and it's best to save them as TGO at a specific library location for easy use in later projects. A lot of folk use Gaea for terrains, but many can be made within TG as well. There's also an erosion plugin, which works great.
have fun!
