You first have to choose an XMPP server that offers you the services you need. One of XMPP's features is that servers provide methods to communicate with contacts on 'legacy' messaging systems, such as ICQ, MSN, .. (called transports).
It's not necessary to have every transport you'll require, because you can use transports on a server different to that which you're connected to. With that in mind, you should concentrate on finding a server that's fast, close to you in location (the two usually go together), and which provides a sufficient set of features. There is a good list of servers on the xmpp.net website. A feature listing of many servers can be found at the conversations.im compliance tester
Another advantage of the XMPP protocol is that you can be connected more than once with an account (multiple devices).
You can specify your personal details in the Personal Information tab, set the account preferences in the General tab, and if you use a proxy, you can configure it in the Connection tab, and finally if you want to use OpenPGP, you can configure it in the Personal Information tab.
You can now add contacts by clicking on Add Contact under Actions menu.