The Jamulus official website will tell you everything you need to know about Jamulus. The Getting Started page will help you to get started.
"Jamulus How to" by Anand Sitaram is a very good introduction on how to use Jamulus. If you don't have time for the full hour, we recommend you at least watch the 23 minutes between 09:20-32:20. At 46:43, Anand and Jonathan talk about building a specific Jamulus computer on a Raspberry Pi, which is part of the solution we offer at MusicBridge.
For singing online, a good quality USB headset works well. We recommend the SADES USB gaming headsets.
For instruments like pianos, cellos and violins, a condensor microphone is a good choice. In this case you will also need to connect stereo headphones to the microphone itself.
For electric instruments such as electric keyboards, synthesizers, electric guitar, or electric bass, a USB audio interface is required.
In this case you will also need a microphone and headphones.