There are lots of channels on Slack you might be interested in joining, such as #prayerrequests, #datainpictures or #funandgames.
To join a new channel, click the plus button next to 'Channels' and select 'Browse channels'.
Search for the channel you want to join, find it in the list and hover your mouse over it, then click 'Join'.
That's it! You can click on the channel name to go straight to it, and it'll now appear in your left hand menu under 'Channels'.
Alternatively, you can click into the channel first to get a feel for it, and then select 'Join Channel' down the bottom if you want to join it.
When searching for channels, those with the # symbol are open to anyone to join. Channels with a padlock are private and can be joined by invitation only.