It depends on your phone or computer which Jabber program you can use:
For Android, the recommended client is Conversations. You can get it in Google's play store (for a small amount of money and it's worth it) or in the free software repository F-Droid (for free).
An alternative for Android is Chatsecure.
With a Windows or Linux computer, you might to try Gajim. Please make sure to install plugins for end-to-end encryption, namely OMEMO (only in available for Gajim) or OTR.
For Macs, Adium is a popular Jabber client.
Why Jabber? We have Whatsapp?
The cool thing about Jabber is that it is not a centralized service. Jabber is not run by a single company. Anyone can run a jabber server and invite people to get an account. This is more failsafe and much harder to surveil.
How to find people?
Just think of e-mail: You'll have a so-called Jabber ID that is composed like an e-mail address (firstname.lastname@example.org). Here, your address will be <your-nickname>@jabber.cat. You have to exchange Jabber IDs like you exchange e-mail addresses with your friends. There is no central "addressbook".
Security and encryption
TLS 1.2 only for client-to-server and server-to-server communication
Server-side message archiving is turned on. This enables your different devices to fetch the chat history if they were offline. Please be aware of unencrypted chats being saved on the server as clear text! Use end-to-end encryption! The archive expires after 14 days. Some clients, e.g. Conversations for Android, let you configure the archiving behaviour (in the account settings).