To use your email and calendar, simply head over to SOGo app from your dashboard. Email, calendar, and contacts are integrated into one interface, so it should be similar to Gmail or Outlook. You can use this website on both your phone and computer!
There's also an alternative email client that uses Roundcube and I like how fast and simple it is. This doesn't have calendar integration however.
I recommend you use Thunderbird on your phone (Android only) or computer (all operating systems supported). It's been well tested with this server and supports encryption. I unfortunately don't have an iPhone, but if you know of something that works well, let me know!
Otherwise, you might have to do some manual configuring by looking at this documentation. Let me know if you need any help.
Calendar should be automatically detected by some clients, but if that doesn't work, go to your calendar on SOGo and press the "…" beside the calendar's name. After that, click on the "Links to this calendar" button.
Copy the "CalDAV URL" to your app. Here's how you do it on an iPhone and use this app if you're on Android.
Let me know if that works!
Note that emails are by default unencrypted which means me or a hacker or the hosting provider or the US government could have access to your messages. Because of this, I encourage you to use Thunderbird and send encrypted email to your contacts.
Even then, there's a lot of metadata leaking when it comes to email. Even with encryption, the people that you're talking to and possibly your operating system and time zone are not private.
So a better option is to use a tool like Delta Chat or Signal, and use email only for non-sensitive things.
You're welcome to move emails from your previous provider to this one. Take a look at Imapsync which will move your emails from one service (as long as it provides IMAP access) and type in the following for the destination mailbox (replace the login and password with your own ofc):