Where can I get alerts for earthquakes in California?

According to the answers to Is earthquake prediction possible? on Earthsciense.SE, no technology currently exists to predict earthquakes with enough accuracy and advance warning to be very useful, nor are we anywhere close to being able to develop it.

There is an experimental system called ShakeAlert being developed at UC Berkeley that was able to give a warning 10 seconds in advance of the severe shaking of the Napa quake on August 24, 2014 (by detecting and analyzing the initial, less damaging waves of the quake that had already arrived). This system is not yet considered mature or reliable enough to make its alerts available to the public, and since it appears to be the cutting edge of research, this suggests that nothing of any usefulness is publicly available.

Hence, the answer to your title question is you can't.

Your best bet is preparedness: knowing how to respond should an earthquake start, having emergency supplies on hand, staying in seismically safe buildings and structures, etc.

There are various services that will send you alerts after an earthquake occurs, but that doesn't sound like what you want.

Basically, living with that risk is part of life in California and other seismically active areas, and like the locals you simply have to get used to it.

If you have a Twitter account, you could follow people from a few key points near you in California. During the recent earthquake in Napa, CA, people started tweeting about the earthquake right when it was going on. Earthquakes travel at the speed of sound, but tweets travel at 2/3 the speed of light. This means that about 100 km away from the epicenter, the tweets about the earthquake will actually reach you BEFORE the earthquake itself.

You won't get much warning (just a few seconds), but that's enough time to get away from glass, chandeliers, heavy cabinets and other stuff that can injure you.

SPECULATION: In theory, you can create an app that uses the accelerometer of your phone to monitor earthquakes as they happen, let them send it to a local server, and from there send out tweets and alerts. I can imagine that such an app is currently in development or even already in use.

In addition to using Twitter, the Federal government also has a special system based on celltowers that sends free messages to everyone in a certain area. http://www.ctia.org/your-wireless-life/consumer-tips/wireless-emergency-alerts explains how it works. You might have to enroll in it though, because your phone needs to be compatible. Contact your local mobile network provider to learn more.