Whisper Systems brings you Signal Private Messenger, an application that you can download for free from the Google Play Store or the Apple Store for your mobile device. It allows you to send encrypted messages without paying SMS fees. It allows you to send media and create group messages as well. All in complete privacy!
When you arrive at the site, I recommend scrolling down the page to learn more about the application and to find the download links. Once you download the application and install it, you’ll need to verify your account by providing them with the code they text you. After you’ve verified your account you can install Signal Desktop, so you can use Signal on your computer. You’ll find instructions to do that here.
Have an issue with Signal? Check out their robust Support section with the link on the navigation strip at the top of the page. If you’re into application development you can also check out their Developers section where you’ll find all the technical documentation for Signal.