WoW: Classic is scheduled for August 27 (technically August 26 if you're in the Americas). The soonest that you can play it is starting in May, during Blizzard's scheduled stress tests. To opt-in you must have an active subscription on your Battle.net account. If you do, go to Account Management and select the Wow Classic beta. Keep in mind, there will be level caps during the test. The schedule is as follows:
Related to this, we’ve seen a lot of questions about the general invite process and want to clarify a few things. The method in which people are invited from the opt-in pool is not on a first-come-first-served basis, but instead we look at a variety of factors such as the age of the account. That said, the amount of closed beta invites that we’re sending out is rapidly decreasing as we have met our current population needs for testing. We’ll regularly reevaluate the need for additional invites as we go through the testing process.
Swim Speed Potions (used especially at lvl 37 for Faldir's Cove in Arathi Highlands and lvl 57 in WPL). Try to get multiple stacks of these. I also recommend keeping a stack of these always in your bags. You will frequently pass rivers and lakes and can use these potions to pass them faster through out the guide. Please note that these potions do not stack with Druid's Aquatic Form and Glyph of Aquatic Form.