While you can start using Jamulus with the microphone that is built into your computer, these small microphones are designed for video conferencing and optimised for speech. They often sound "tinny" and can pick up a lot of unwanted sound, especially if you have a computer with a noisy cooling fan.
Upgrading your microphone can improve the clarity of your playing so the group can hear you better! It can even make it easier to configure Jamulus too!
However, cheap mics (£30 or less) tend to provide little improvement in sound quality. More "professional" mics can be a significant investment of £100 or more, but some do not include features that are important - or at least, useful - for Jamulus.
The microphone I've listed below is the first I've found that is frequently available for under £100 (£70 at time of writing) and includes everything I'd look for in this type of mic. Certainly not cheap, but I think this represents good value for money.
I have confirmed it works well - and is easy to configure - with Jamulus.
iPad users may need an adapter - see below.