Firstly, you've not posted in the correct section, I expect an admin will move the post to the RACE07/Mods section.
If you have RACE 07 by Steam, you will need to be in C:/Program Files/Steam/steamapps/*yourusernamehere*/race07
You then download a mod, track, car etc and open it with WinRAR, link here. When you open it, it should have a folder called Gamedata and maybe another one called UIData. Add these in the directory above. Click yes to all when Windows prompts you about adding/replacing files.
Then start up RACE 07 and select the mod/track/car in the menu as you would do normally with the default content.
If you don't have RACE 07 on Steam, you won't be able to play online in the racing club and the RACE 07 directory will be C:/Program Files/SimBin/RACE 07.
Hope this helps, any questions just ask, I'm sure everyone will be happy to answer them.