Since you asked about Renton, there are typically 25-30 players, with about half in the 16 to 22 age range. If there is a big tournament in the area, league generally will not meet. (Next big tourney will be late September.) The shop owner is pretty cool. Just show repect by keeping the play area clean, no swearing, etc.
During league, it is pretty much up to you to find someone to play. There are lots of friendly folks who are willing to play and can teach you the latest rules changes. You get two stamps for a win, one stamp for a loss. When you accumulate enough points, you get freebees - promo cards, special holographic energy, "badge" pins. One league season is wrapping up, and a new one will start in a couple of weeks, with all new goodies.
Most leagues run "unlimited" play, meaning you can use any card from any set, although most players will be playing the newer cards as they get ready for tournaments, and mess around with new ideas for decks.
Hope to see you at league sometime!