Then you are going to have to buy the most recent installment of the Command and Conquer 3 series: Kane's Wrath. The process seems pretty straightforward - buy the game, look for an insert, follow the directions, and WHAM you are in the beta queue. Depending on your location and position in the queue, the download will be made available to you at some point after it goes live in priority order.
Check it out for yourself: "Within every purchase that contains the Red Alert 3 Beta Key Sticker is a key that guarantees you access to the Command & Conquer Red Alert 3 Beta program if you register before September 15, 2008." Every single copy of the game in North America will have a beta key insert, and if you are unsure of your country's participation, make sure to inspect the packaging for an obvious RA3 callout. If it is on the packaging, then it will have the insert.
I haven't played any C&C since Generals, but I am itching to get my hands on this game. So far, the only way to gain access to the beta queue is by purchasing another fully-priced game; I have waited this long, I can wait until it is released.