Hello mondytd, usually tracks you have to unpack in the root GTR Evo folder. For example if the file you have downloaded has the folders 'Gamedata' and 'UIData' then unpack those in the root folder. If however the folder is called 'Locations' then it must be extracted in the GTR Evo/Gamedata/ folder. Most tracks have the instructions in them, if not just ask around someone is bound to help you.
Downloadable cars? I don't know what you mean here, maybe like mods ie. F1 Mod, Escort mod. These also have instructions and usually have to be installed in the root GTR Evo directory as well.
If however you mean existing cars with different paint jobs/skins then the go into My Documents/Simbin/Race07 or GTR Evo (whichever one you have)/Custom Skins.
Hope this helps.
Also FYI, you should have posted in the general or help section of this forum, that way you would have gotten a much faster response, please advise next time .