It is possible to use natural rock and wood you find in nature although I highly urge you not to for your first tank.
Lets break this down a bit because it can get overly complicated. It is possible to use natural rock and wood you find in nature although I highly urge you not to for your first tank. While most river rocks are perfectly aquarium safe, other rocks that you find in non aquatic environments could cause issues. More so, the same can be said for wood or driftwood. While highly unlikely in North America, natural driftwood found in a stream could contain algae or fish pathogens and may require you to boil and sanitize before use. Some types of terrestrial found wood may have saps that can cause problems in your tank. Save the el natural strategy for a future tank.