Get it on the App Store: https://itunes.apple.com/us/app/signal-private-messenger/id874139669?mt=8
Read the how-to guide from EFF: https://ssd.eff.org/en/module/how-use-signal-ios
Get it on the Google Play Store: https://play.google.com/store/apps/details?id=org.thoughtcrime.securesms
Read the how-to guide from EFF: https://ssd.eff.org/en/module/how-use-signal-android
And they are working on a Desktop/Web version: https://whispersystems.org/blog/signal-desktop/?ref=2677989.
Do you want to learn more about how to protect yourself from ubiquitous online surveillance? The EFF has put together this list of tips, tools and how-tos for safer online communications at https://ssd.eff.org/.