My high definiton is working great and everything is fine except one major thing, which is that there is no sound at all. I would like to know how i might fix this problem.

Good Day, timerwer;

HD is fine but no audio! Not knowing that if you ever had sound, have surround system, hooked to an input or RF, I would suspect that you only have the Green, Blue, and Red cables hooked to your TV and the Audio cables ( red - white ).
If you do; unplug TV (reset, reboot) from A/C outlet for 3 to 5 minutes. If still no work try these suggestions:
1. Make sure the TV's internal speakers are turned 'ON'.
2. Make sure the "BOX" ( cable, sat, u-verse) volume is turned up all the way up.

Good Luck, big IRISH.