You should be able to go to the community browser (between messages and home) and just search for a community, for example "android". It will then list communities that fit the search term sorted by number of subscribers.

If you see for example a community just called "android", it means that the community is on the same instance as you are logged in. If you see something like "[email protected]", it means it's on a different server/instance called "lemmy.ml".

It doesn't really matter if you join a local community or one on a different server, so I would recommend you join the one with the biggest subscription numbers.