This league is pretty much full with 18 players, although if another person shows up who wants to play I'd be willing to bump it up to 20. The BigDog or Avis league might still be looking for people, though, I'd read through those threads to find out.
Since I see you already have a Yahoo account from the Pick 'Em thread, to play fantasy basketball you just join the league on Yahoo after the commissioner gives you the ID and password. You can also join a public league with random people, which is probably a good idea to get your feet wet assuming you don't get in here or in one of the other leagues here that hasn't drafted.
As far as how to play, basically each fantasy player drafts a "team" of NBA players. Each NBA player can only be on one fantasy team, so part of the skill is deciding who to grab for your team at the start of the year. Then each fantasy team competes based on the stats of the NBA players. These leagues are head-to-head, so that means each week your fantasy team plays another fantasy team. The stats for the players on each team are accumulated and compared, and you get a point for each category where your team's points are higher. For example, I might play Bricklayer's team in the first week. Since Bricklayer likes big players, maybe his team has more rebounds and a higher field goal percentage than mine, but mine has more assists, three pointers and free throw percentage. So I would win that week 3-2 (there are actually 9 or 10 categories in our league).
So each week you decide who starts and who is on the bench for your fantasy team. Maybe the Spurs only play two games all week so you sit Duncan, or maybe the Kings are playing at home against Cleveland, Memphis and Golden State and Carl Landry is injured so you start Jason Thompson. YIou also need to watch out for players that nobody drafted in the beginning but that play well, and replace players on your team who are injured or don't play as well as you expected. I'm starting to get into too many details here, but hopefully you get the gist. It's a lot more complicated than the pick'em game, but a lot more fun as well.