Maya's interface can certainly look overwhelming at first, but it's actually quite simple and easy to understand. It's just that Maya is capable of doing so many things, it can be difficult to know where to begin. Maya's actually more acurately described as a platform than a program. It has thousands upon thousands of functions. It can do pretty much anything.
That having been said, it's not ideally suited for SL animation unless you already know what you're doing with it. It's kind of like swatting a mosquito with a bazooka. Sure, you can do it, but unless you've got a few other things you plan on doing with that bazooka as well, it's kind of a waste of ammo for such a small task. Don't get me wrong; I personally would rather use Maya than the alternatives, but that's only because I also use it all the time for other things so it's natural for me to grab it for all things 3D. Were my only concern SL animation, I probably wouldn't bother learning such a gigantic application as Maya when far less complex programs like Poser and Daz Studio are so pre-purposed straight out of the box for exactly this type of thing.
I don't mean to discourage you from getting to know Maya. It's easily one of the best computer programs that's ever been written, and you can do tremendous things with it. Just be aware that since humanoid character animation is such a minute fraction of what it can do, learning to do just that one task with it while bypassing everything else can be pretty needle-in-a-haystack'ish. Because it can do everything, discovering how to tell it to do any one thing can take some patience to learn. Like I said though, it's not hard; it's just that there's an awful lot there.
If you do want to use Maya, the first thing I'd recommend after you install it is to watch the training videos that come with it. They will familiarize you with the basics of the interface. Also, make sure when you install it, you include installation of the help server, and make sure your firewall is set to allow Maya to access it. Even though the server gets installed locally (and only runs while Maya is running), firewalls tend to think it's the internet, so Maya will need to be unblocked in your firewall configuration.
The help is excellent by the way, hands down the most well written help files of any program I've ever seen. I'd recommend after you watch those videos that you work through the first few tutorials in the help to cement what you've learned. They're quick, easy, and fun. You get to have a lot of those "Hey, look what I just made!" moments as you go through them.
As for SL animation in particular, here's where things start to get a little trickier. As I said earlier, Maya can do anything, but the trick is knowing how to get it to do the specific thing you want. As far as that goes, I've got some good news for you, some potentially bad news, and some potentially worse news.
The good news is Orion Neville created a really great rigged skeleton for Maya that you can use to create SL animations quite easily (provided you've already learned your way around the basics of the Maya interface). It comes with a shelf plugin (shelves in Maya are the tabbed button bars towad the top of the screen) to automate the BVH export process for you. It's pretty slick.
The bad news is it was created for Maya 7, and you've got 6.5. I'm not sure if it will work with 6.5. I know it doesn't work with 6.0, and since I never upgraded to 6.5 (went straight to 7), I have no way to test it on the version you're using. You'll have to follow the installation instructions and see for yourself. It will either work or it won't. You'll know right away. It will give you an error message when you start Maya if it can't read the shelf plugin properly. If it can't do that, then chances are it won't be able to read the scene file either (the scene file is the .mb file; it has the skeleton in it).
So, that brings us to the potentially worse news, which is if Orion's rig doesn't work for you, you'll have to make your own. That means learning to create a skeleton (easy), learning to rig it (intermediate level), rigging it so it's the same as the skeleton in the SL avatar (challenging), and finally, learning MEL (Maya Embedded Language) so you can write a script to get Maya to export BVH files in the proper format for SL (hard, unless you're already an experienced programmer). All of that will require quite a bit of learning.
Sorry if that sounds discouraging. As I said before, Maya's a wonderful program, and if you're interested in it, you should absolutely learn it. It's well worth it. However, if your only reason for persuing it is to use it as an SL animation tool, frankly, froma cost/benefit standpoint, it's really not worth the investment of time and effort you'd need to put into it to learn how to make it do that. If Orion's rig works with 6.5, or if you can get ahold of another one that does, great. Otherwise, I wouldn't recomment using Maya just for this.
As much as I hate Poser for being such a (for lack of a better term) dumbed down, one-trick pony, it works just fine for what it is, and SL animation is right up its alley. DAZ Studio is another program that will also work, and it has the added andvantage that it's free.
I hope this info was helpful. Good luck.
