Welcome to the world of FSX and its crazy amount of addons & tweaks!
Here are a few good threads/links to start you off
Native Freeware Aircraft
Freeware sceneries list
Ozx scenery Addons
That should be a good start
id suggest picking an area that you'ed like fly in/about & see what's kicking about to improve it rather than downloading a ****load of stuff and throwing it in all at once,plus some stuff can break other stuff so id go one at a time and test with installs and make sure you always read the readme's !