Do what thou wilt shall be the whole of the Law.
I came across this thread while doing some research online and figured I'd sign up and help answer your questions. I've been a practicing Thelemite for about seven years now and have been involved with O.T.O. and A.'.A.'. for about four of those years.
Thelema is a religion that can be summarized by the words "Do what thou wilt shall be the whole of the Law," which are the words with which we greet others. These words do not simply mean "do whatever you want," but instead refer to the True Will, which is the deepest expression of your innermost self. Every human being has a True Will, a formula for living which, if discovered, enables them to be the best "them" that they can be--to live a life of liberty, fulfillment, and joy.
The True Will is something that is completely unique to each individual. Nobody can tell you what your True Will is--you have to discover it for yourself. Thelema provides a number of very powerful spiritual techniques that enable you to discover this Will by obtaining deep levels of self-knowledge and self-discipline.
The reason I mentioned above that the True Will doesn't simply mean "do whatever you want" is because there are things in which we engage, such as addictive or conditioned behaviors, that do not express our deepest sense of self. Over time, the process of self discovery will reveal these things to you, and you'll have to make the choice whether to take the easy way out and continue with these conditioned behaviors, or to replace them with ones that will make you more happy and free in the long run. For example, some people compulsively spend all of the money in their pay check as soon as they get paid. They have a lot of fun partying and buying nice things. However, if part of their True Will involves buying a house, they will have a hard time doing so unless they learn the discipline to moderate their spending enough to start setting aside money.
You may have guessed by now that the discovery of the True Will is the main focus of Thelema. Since this is the case, and because we believe that it is impossible to know the True Will of another, we value maximum personal freedom as a central tenant of our tradition. This, of course, inherently implies some limitation in behavior: your freedom to punch ends where my face begins.
In terms of practice, Thelema uses a combination of both ceremonial magick and mysticism. The mysticism comes primarily in the form of Raja Yoga, with some adaptations to be more suitable to modern times. Thelemic magick is syncretic in nature. It is a synthesis of things from the Golden Dawn (of which Crowley was a member), correspondences derived from ancient grimoires, and many other systems with which Crowley was familiar. If you're very familiar with ceremonial magick, you may recognize a lot of things in Thelemic magick because most modern practices trace back to Crowley in some way. Check out Magick: Book 4 (colloquially called "The Big Blue Brick") for a pretty comprehensive look at Thelemic magick and mysticism. It's not exactly a cheap book, but it is one of the best books in which you can invest, Thelemite or not, for studying magick.
The best way to really get started studying Thelema at a serious level is to get involved with O.T.O., a Thelemic fraternal organization with local bodies all over the world. O.T.O. provides an initiatory system and a social framework in which you can experiment with the Thelemic values of individual liberty and self discipline. I'm not sure if you're from the U.S., but if you are, you can find the nearest local body here:
http://www.oto-usa.org/bodies.html I would like to say that joining O.T.O. is not a requirement for being a Thelemite, or even a serious one at that. It is however, highly recommended. If you give it a chance, the initiations alone will completely transform your life, as will the people. Some of the strongest and most interesting people I've ever met are people I originally met at an O.T.O. event.
If you'd rather do some reading on your own, there are a few documents I'd recommend. For learning about Thelema itself, check out:
* The Message of the Master Therion: http://hermetic.com/crowley/libers/lib2.html
* De Lege Libellum: http://www.sacred-texts.com/oto/lib150.htm
* The Book of the Law: http://www.sacred-texts.com/oto/engccxx.htm - This is our central holy book. It won't make much sense at first, but when studied over a period of time, you'll start to see the profundity of what it is saying.
* Crowley's commentary on The Book of the Law: http://hermetic.com/legis/new-comment/
* Duty: http://lib.oto-usa.org/crowley/essays/duty.html
* On Thelema: http://hermetic.com/eidolons/On_Thelema
That should keep you busy for a while! However, if you're interested in delving more into Thelemic magick and mysticism, check out the following for beginner's material:
* Liber E: http://www.sacred-texts.com/oto/lib9.htm
* Liber O: http://hermetic.com/crowley/libers/lib6.html
* Magick Without Tears: http://hermetic.com/crowley/magick-without-tears/
* Eight Lectures on Yoga: http://hermetic.com/crowley/eight-lectures-on-yoga/
There is much more advanced material out there, but you will cheat yourself in a major way if you don't do the beginner's stuff first. While some of it may look trivial, it actually has an extremely potent effect on your personal development. Don't believe me? Work your way up to an hour a day doing the Pranayama section of Liber E. By the time you master that, you will have had some of the most intense visions and entered some of the most intense trance states you've ever experienced--on par with the potency of psychedelics.
I hope this answered your questions and I wish you the best of luck in your spiritual journey. If you have any further questions, don't hesitate to get in touch with me by using the contact form at http://ararita.org/
Love is the law, love under will.