Feel free to post in !newToLemmy@lemmy.ca :)
We also have a guide on finding communities pinned on !communityPromo@lemmy.ca
As for your question:
I use lemmyverse.net to find a few communities. Usually one is actively used and the rest are dead. If a few have recent posts, I'll subscribe to them all to get the content. Later you can post to all of them, or the one you like more (friendly moderators, nicer discussions, etc.)
Unfortunately to see total subscriptions, you either need to go to the instance itself or wait for 0.19 to fix the issue where it shows the lower count (number from your instance only)