Newbee: where to find user instructions?

Just installed Spark 2.8.3 on Debian Stretch with JRE 12.0.2 (1.8 is also on the computer but not the default). I could not find any user documentation - where to look?

My goal is to have a desktop client for communication with Conversations Android client using my Jabber account. However, I am unsure whether choosing Spark was the best choice (I am fan of Java) for this purpose. Especially I do not understand why I am asked to create an account - I already have a Jabber account - and how to use my existing account.

The only piece of user documentation is the user guide here. Which you can also reach from Help > User guide menu in Spark.

When you run Spark for the first time it shows the login screen, where you can put your existing account credentials. If you go to Accounts menu, there you can create new accounts (this is optional).

Spark indeed uses Java, but it uses rather old Swing technology and is almost abandoned currently (no developers, only occasionally contributors). You can check what other XMPP clients are on Linux https://xmpp.org/software/clients.html

Btw, Spark officially supports only Java 8. It runs on newer Java, but some components might not work (plugins) and there can be some weird behavior. Maybe new account screen popping up was one of them.