Hello:
I would start out by downloading both Real Airlines, usa and europe from simviation.com: http://www.simviation.com/fs2004scenery52.htm
Those two downloads will increase your traffic activity. When you down load these, make sure you specify where your FSX is installed. Also after you down load these files, go to you FSX root directory and transfer all the planes inside a folder called aircraft to the simobject/airplane folder.  If you find that that this is too difficult for you to do, then I would suggest to get a payware traffic X that will install all of your traffic files for you.