I'm going to post them anyway:
1. Do not power on your phone. If it is powered on power it off.
2. Activate your SIM card: Dial (877) 807-4646 from any phone and follow the prompts to activate your 4G LTE service.Have your Customer Receipt with you because you may be asked to confirm information from it, including the mobile number assigned to your 4G LTE SIM card.
3. Do not power on your phone yet.
4. Insert your SIM card and battery into your SIII.
5. Power it on.
6. When prompted to use Backup Assistant, say NO. BA is unnecessary bloatware. Welcome to the world of Android where all your contact info sync and gets pulled from Gmail. The General Help and How To Forum has stickies on how import contact info from almost every device imaginable. I also posted a sticky in the Introductions forum that includes links on getting started if you're new.
If you've already activated without the power off requirement -- good for you, but not doing so has caused me issues in the past.
If you already have a micro SIM from a prior device you can skip the above steps. Just remove from the old phone and stick it in the new one.