The first thing you will need to do is purchase the game, obviously. The game comes in two editions - the Standard Edition, which just includes the game, and the Collector's Edition, which includes the game in a limited edition case, a statue of Diablo's head, Diablo 2 on a crystal shaped USB flash drive, and lots of other goodies, such as an art book and the soundtrack.
You can get the Standard Edition in retail stores or digitally off Blizzard's official site (https://www.battle.net/account/management/digital-purchase.html?product=D3).
If you're after the Collector's Edition you're going to have a harder time since they have sold out, though you may be able to find it in stores on May 15th.
Once you've purchased the game, Blizzard allows you to pre-download the game client, so you can be instantly ready to go when the game servers go live at 12:00 AM (Midnight) PST.
2. Create Your BattleTag
Blizzard requires everyone to create one of these special ID's to play Diablo III. They expect that their servers are going to be bombarded when the game is released so it would be a good idea to set yours up in advance on the Battle.net site here:
Be sure to make it something you like because whatever you choose is permanent and cannot be changed later.
3. Call Off Sick At Work
If you've been saving up vacation at work, now would be the time to use it. If that isn't an option, it's time to brush up your acting skills, so you'll be ready to call in sick come Tuesday. Just hope your boss isn't a gamer too! :-D
4. Prepare Your Diablo III Survival Kit
Head to the store to stock up on supplies you will need. Some staples of the gamers diet include Doritos, Hot Pockets, Ramen, and Red Bull. You'll need plenty of nourishment in order to stay up all night slaying demons. Don't forget the toilet paper!
5. Get the Diablo III here Guide
This one is a must have. The experts over at Diablo 3 Here Guides have created the end-all guide for mastering every aspect of Diablo III. These are the same guys whose famous Leveling Guides revolutionized the way people play World of Warcraft. Their all new Diablo III guide features a full walkthrough of the entire game, class and crafting guides, followers optimization, achievements, and more. Anyone who hopes to beat Diablo III, especially on the painfully tough Hell and Inferno modes, should pick up this guide before launch day.