Press the communities button located on your bottom toolbar
Navigate to the “Suggested” communities tab to see some communities that display thinks you might be interested in.
If you see a community you like, simply press the “Join +” button. If it is a public community, you will be accepted immediately. If it is a private community, you will have to wait for an admin to accept your request. There are also “Exclusive Communities” which are by invite only.
Alternatively, you can also search for communities using the search function. Type in something you’re interested in, and if communities based around it exist, they will appear in the search results.
Press on the community within the search results, this will bring you to the community page, where you can then join.