Its easy for the GS3, compared to some. But if it's your first time, read carefully. With root you get access to your system. This is needed for theming, mods, or to use apps that need access like a WiFi Hotspot app, or Titanium Backup. Unless you want to learn about Roms and Ginny pig your phone, keep your phone as is and just root if your going to.
Place to start...
http://forum.xda-developers.com/showthread.php?t=1725395
[HOW TO] SGS3 LTE CWM Recovery and One Step Root
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