Find "Signal" in the Google Play Store and install it. Allow it to access "contacts and media" and "make and manage phone calls" when it requests permission. It will ask for your phone number, but should be able to detect it automatically. It will then send you a verification code, which it may be able to detect automatically; after that you can set up a profile (i.e. enter a screen name for identifying yourself to other Signal users). This will default to your Google user screen name. You can also allow Signal to be your default SMS/MMS app; this is not required, and can always be enabled later.