If you are talking about some brand new instance that you just created, you can construct a URL with the community name to find it the first time, then someone from your instance needs to be the first to subscribe/join it. After that, someone just needs to search for it using its name from the Communities link at the top of your instance homepage - making sure that All rather than Local is selected at the top.

PieFed will pull in old posts, I think Lemmy won't, but either way the old comments and votes are a lost cause, however newer ones (after possibly a bit of a delay, maybe 24 hours unless there's a larger backlog of things to sync) will automatically sync up from then on (additional caveats exist like locked/removed posts).

So for example to get to the !AskUSA@discuss.online community, you'd go to https://yourinstanceurlhere/c/AskUSA@discuss.online and join (e.g. https://lemm.ee/c/AskUSA@discuss.online is a functional link).

Caveat: the instance (Discuss.Online in the aforementioned example) must already be federated with, which you should see in the https://yourinstanceurlhere/instances, and not also defederated with (lemm.ee barely defederates from anything, but most instances have a much more substantial list of spammers and gore and bad-faith instances that they curate so their users don't have to do so individually). An easy way to get there: go to homepage, scroll all the way down and click Instances; to see the defederated list click the Blocked Instances tab at the top of that.

But from your current instance lemm.ee, all of this has almost certainly been done for you, as iirc it is the #3 largest instance across the entire Fediverse. Just make sure that searching the Communities list is set to All rather than Local!:-)

Edit: but as for how to discover the community names in the first place, simply browse some existing instance like lemm.ee, you don't even need an account to see most of them (those not marked as internal-only).