Yeah welcome to our rather small online community.  Usually during prime time there will be about 140 people online with most of those already playing a game, 60-90 in afternoons and 30 in the morning.  Clans aren't so popular as friends list are so start updating yours when you meet people you like.  It seems like most of the casual players stick to regular Sins, while the more competitive ones play Entrenchment.  Either way I'd still recommend getting it because at $10 it is a steal.  Don't bother with dimplomacy yet because it is just a beta and virtually no one plays it online.

Comp stomps are a great way to find the other casual players, although alot of these people play in set groups and password protect their games.  If you join these games through the chat channel function (chat is always the same as game name) they are usually nice and will let you play if you ask. 

The more competitive games are the 4v4's and 5v5's.  Jump right in if you want but I would recommend playing the game for a bit and getting good with at least one of the races before jumping into these.  You'll see some 2v2's and 3v3's and these can be a mixed bag.

If you are wanting to learn there are IMO 2 good ways of doing it.  The first is to record games.  If someone completely outplays you before you leave the game go to the in game menu and hit the record button and save the game.  Then in single player watch the game from their perspective and see wht they did better/differently the you.  Secondly learn the ins and outs of the game.  Learn why your 30 cobalts get eaten alive by his 20 illuminators, and then when you build 10 carriers to deal with the illuminators they get destroyed by disciples which are equivelent to your cobalts.  The stratgey forum is a great place to browse and asking people online to play a game with you specifically to help you learn is also a good idea.