After you successfully registered your account, you can either use the Web Client or better use a dedicated XMPP client. Here is a list of recommended clients:
|IOS||Siskin IM||App Store|
There are lots and lots of other XMPP Clients on all platforms. Some may be outdated or not supporting all features for todays needs such as OMEMO. The above list is a result of common recommendations.
Client list was taken here, thank you! :-)