Apple has opened up the beta program for the first million people who download the public beta of OS X Yosemite 10.10. Go to this page and sign in with your AppleID. You will need to agree with the terms and then it will load a redemption page with a code on it for the Mac App Store. Clicking on that will load the Mac App Store and start the Yosemite beta download.
You do need to have 10.9 Mavericks running, 2GB of memory, and 8GB of drive space available.
The terms of the agreement state you can talk to other people in the beta or developer program about Yosemite and you can talk about things that Apple has announced, but otherwise you can’t talk to others about the beta.
And if you get stuck with a message in the Mac App Store about the code has already been redeemed, go back to the beta program redemption page and refresh it. It should pull up a new code.