You need two parts: the Android App (apk link; Android Market link) and the server app running in your house. Further, the server app needs to be connected to X10 hardware to toggle your garage door.
For the server software and docs on compatible hardware for your garage, see:
http://github.com/bradfitz/android-garage-opener
Notably:
Once you install you're running the server, configure its URL on your phone as http://your-hostname-or-ideally-ip-address/garage. Note that the endpoint by default is /garage and the root "/" path is just a placeholder, in case you want to proxy the garage webserver behind another existing webserver on your home IP address (or dyndns hostname).