The obvious first step to get OpenHAB on your Android is to install the OpenHAB app from the Google Play store.
When you first open OpenHAB it is running in demo mode, so let's change that by changing some settings.
Only two things you need to worry about now: set the OpenHAB URL and uncheck Demo Mode. If you have your router configured to allow outside access to your Pi, you can set the Remote URL as well.
Type the full (local) URL to the OpenHAB site.
And that's it!