According to SamMobile, the update will first roll out to Poland, followed by more countries later in the week. But those who are impatient can install the update via Odin by following the instructions on this website.
Jelly Bean has been available in various forms for Galaxy S3 owners for a while now, but the official version of the update is just now being announced. The benefit to installing Jelly Bean via the official update is that you don't need root access, and thus won't void your warrantee if anything goes wrong. It's also a lot easier to install an update via Kies than via a custom ROM.
Make sure you download the update via WiFi and that your battery is at least 65% so the update won't be interrupted.