If your PC do not know your phone, Install your phone driver on your PC or install Kies. If you already install driver, forget it.
Power off your phone
Enter download mode by pressing and holding HOME+VOLUME DOWN and then power button (press them in order).
Doing so will get your phone into download mode.
On the screen, the letters with ! Sign will appear.
Accept notice by pressing Volume up button.
Now you are in the download mode.
Open Odin on your PC.
Choose PDA on Odin.
Choose firmware you have downloaded.
Connect your phone to PC with USB cable.
Search letter "ID:COM" at the left upper block of ODIN.
If this letter appear, you are successfully connected and start downloading.
Do not forget to tick NAND ERASE ALL.
After download finish, you phone will reboot automatically.
Be sure to check you are downloading right firmware.
If you are not sure, search on forum or google how to flash firmware with Odin.
Forgive me that I cannot show you complete instruction because I am replying from Mobile.