Here you go sir.
Root :
http://reviews.cnet.co.uk/mobile-phones/how-to-root-your-samsung-galaxy-s3-50008588/
Download CM10.2 :
http://download.cyanogenmod.org/?type=stable&device=i9300
download Gapps:
http://goo.im/gapps/gapps-jb-20130813-signed.zip
Xda link :
http://forum.xda-developers.com/showthread.php?t=2382573
Copy both of those into the root folder of your phone.
Once rooted, boot into recovery mode by Home & Volume Up & Power.
Once the device boots into the ClockworkMod Recovery, use the physical volume buttons to move up and down. On most devices, the power button is used to confirm your selection, although for some devices the power button is used as a "back" button to go up one level, in which case the home button is used to confirm the selection.
then (goto advanced and select "Install update from SD Card")
- Flash CM10.2 zip from SDCard
- Flash GApps zip from SDCard
- DO A DATA WIPE / FACTORY RESET (otherwise your device will be stuck at boot) -- this is done in recovery mode.
- Reboot