How do i make terrain

I want to make a forest (like in battlefront(endor) the witcher or crysis3)… I can extract models of trees and bushes from some games(I know and this is only for personal use)
Where do i find grass… How do i make textures so that they look professional, where do i find these textures…
Please help

Also… i am making a dungeon… it is in parts: corridor 4-way corridor stairs… how do i make the rock textures match between parts but not repetitive? example: the same rock appears every 2 meters

these are all relatively basic things that there are a million tutorials on. do a quick search and youll find somethihng