Go to Communities and select All. This is a list of all the communities that were already looked up by at least 1 user on the instance.
I think it's a good starting point that will lead to discovering other communities over time that reflect your personal interests.
Brief overview
Your account is tied to an instance
You reach all other instances via the instance your account has been created on
You can post in any communities that have not been banned by your instance
You can comment on any posts in any communities that have not been banned by your instance
You can only create communities in the same instance as your account, assuming the instance allows new community creation
General usage
Note: if you're the first user of the instance looking for a particular thing - it can take a ridiculous amount of time to get a result. In regards to communities I've found it quicker to just search for them and then go into Communities/All tab to find them there.
Looking for a community
Click the Magnifying Glass in top right to search.
Enter the community and instance (I'm using general as community and lemmy.cafe as instance in this example) into the search field in one of the formats: