For starters, you will need to get the game first – which is a no-brainer. Pick it up straight from Blizzard through the digital presale, or place a preorder at major retailers and other gaming outlets. The potentially sought after Diablo 3 Collector’s Edition will remain a physical retail-only copy, so forget about looking for it online save for scalpers who might want to make a quick buck off you.
Other than that, remember, too, to create your BattleTag, and bear in mind that this non-unique nickname is a permanent choice, so choose wisely. Be aware of time differences due to regional launch times applied, so the launch time has been broken down according to its region. There will be an early install ability, since the game’s installer will be unlocked at 8:01 a.m. PDT on May 14th, letting you install and troubleshoot the title, if necessary, before you download and install the launch day 1.0.1 patch.
Good luck, brave adventurer! You might want to check out the Diablo 3 Hero Planner while waiting.