You can earn a decent amount of XP by doing Herbalism or mining. Any zone works but I prefer Sholazar Basin as a lot of the mobs are not aggressive and the mobs are spread out so you will find a lot of herbs and ore that you don’t have to fight for. It is possible to make about a level every hour by doing this, or a level every 30-40 minutes with rest XP.
The most important thing you should note is that there are no official beta keys. Blizzard opens the gates to their betas by granting access to your account — not sending you a beta key. If you’re in the beta, you’ll be able to download and play the WoW Classic client from your Battle.net launcher. If not, you aren’t in the beta. It’s easy to be tricked into thinking you’ll get a beta key from an email that looks like it’s from an official Blizzard address, but don’t believe their lies — they’re all fake.