This is how I got my game to work on windows 8 (hopefully it will work for you):
1) Download this and extract the files to your Tiberian Sun folder http://www.commandandconquer.com/forums/index.php/topic/26975-running-tiberian-sun-on-windows-8/
2) Press right click on the game or any shortcut, press properties and then go to compatibility (at the top).
3) Check run as administrator and try the game.
4) If the game doesn't work or crashes (which is most likely to happen) run the program in compatibility mode for windows XP service pack 3 (in properties and compatibility as well).
5) Run the game again if it still doesn't work (which I doubt) press reduced colour mode and change it to 16-bit (in properties and compatibility as well).
I really hope the game works for you, good luck .