Go to a site that offers downloads for MS Flight Sim and pick something you like and download it.
http://www.fsfield.com/ has good stuff and does not require you to register.
There is almost always instructions included on how to install the add-on and it rarely involves more than copying the files to the correct folder in the MSFS folder in Program Files.
One very worthy add-on (or many actually) is got at
http://www.projectai.com/ which will fill the skies with real world airplanes running to realistic timetables.