It depends what tier you want to play, especially since it could be more accurate to say there are competitive scene
s and communitie
s (in general one around each tier). One way to get more involved into the competitive scene of a given tier is to participate in their tournaments, which are often announced on their Pokemon Showdown room or on their discord server - you can find a directory for Smogon discord servers
here (and one specific to NatDex metagames
here). Most tournaments that happen on the website are announced in the official Smogon discord server too which you can find in the first link. In general, nowadays a huge portion of the activity happens on discord servers and joining one of those is probably the easiest way to get into the stuff
To get more involved into the community of a specific tier, in addition to the discord servers you can find dedicated subforums on the front page where lot of the activity happens:
"Smogon Metagames" regroups pretty much any current gen metagame on the website that are not VGC or BSS. Note that a couple of those sections are actually home to multiple metagames, for ex Unofficial Metagames is where you will find ZU or Ubers UU for example.
Ruins or Alph are dedicated to old generations, RBY being the first and SS being the 8th.
Finally, Battle Stadium is dedicated to Nintendo's formats, so VGC and BSS.
Hope this helps, if you have any other questions feel free to ask!