CS:GO beta keys were given out at two past gaming events, EuroGamer and PAX.
There is currently NO WAY to get a CS:GO beta key
, unless you get your hands on one of the keys from the aforementioned events.
The developers of CS:GO have told us that an open beta is coming in the new year. Bookmark the forums and follow @csgo_dev on twitter and you will be notified when there are keys available or an open beta is starting.