To find services you want to use, you need to discover them first. This can be done with either service discovery or browsing. Service discovery is the newer and preferred protocol, while browsing is still used by much software. The use of both is very similar.
The most common use of service discovery is to browse your home server, to see what services are provided locally. Note, however, that this is no restriction; you can use services from all over the network.
To start browsing, type M-x jabber-get-browse and enter the JID
you want to browse. For service discovery there are two commands,
jabber-get-disco-items and jabber-get-disco-info,
depending on whether you want information about that specific JID or
about services related to it, respectively.
These commands can be accessed from the Info menu, which is opened by typing C-c C-i.