Welcome. As intro pieces, I really like Making Beliefs Pay Rent and Belief in Belief. The rest of the Mysterious Answers sequence is attempts to illuminate or elaborate on the points made in those two essays.
I was less impressed with "A Human's Guide to Words," but that might be because my legal training forced me think about those issues far before I ever wandered here. As a brief heuristic, if the use-mention distinction seems really insightful to you, try it out. If you've already thought about similar issues, you could pass on it.
I think the other Sequences are far less interestingly novel, but some of that is my (rudimentary but still above average for here) background in philosophy. And some of it is that I don't care about some of the topics that are central to the discussion in this community.
As always with advice like this, take what I say with a substantial grain of salt. Feel free to look at our wiki page on the Sequences to see all of what's out there.
If you've recently joined the Less Wrong community, please leave a comment here and introduce yourself. We'd love to know who you are, what you're doing, what you value, how you came to identify as a rationalist or how you found us. You can skip right to that if you like; the rest of this post consists of a few things you might find helpful. More can be found at the FAQ.
A few notes about the site mechanics
A few notes about the community
If English is not your first language, don't let that make you afraid to post or comment. You can get English help on Discussion- or Main-level posts by sending a PM to one of the following users (use the "send message" link on the upper right of their user page). Either put the text of the post in the PM, or just say that you'd like English help and you'll get a response with an email address.
* Normal_Anomaly
* Randaly
* shokwave
* Barry Cotter
A note for theists: you will find the Less Wrong community to be predominantly atheist, though not completely so, and most of us are genuinely respectful of religious people who keep the usual community norms. It's worth saying that we might think religion is off-topic in some places where you think it's on-topic, so be thoughtful about where and how you start explicitly talking about it; some of us are happy to talk about religion, some of us aren't interested. Bear in mind that many of us really, truly have given full consideration to theistic claims and found them to be false, so starting with the most common arguments is pretty likely just to annoy people. Anyhow, it's absolutely OK to mention that you're religious in your welcome post and to invite a discussion there.
A list of some posts that are pretty awesome
I recommend the major sequences to everybody, but I realize how daunting they look at first. So for purposes of immediate gratification, the following posts are particularly interesting/illuminating/provocative and don't require any previous reading:
More suggestions are welcome! Or just check out the top-rated posts from the history of Less Wrong. Most posts at +50 or more are well worth your time.
Welcome to Less Wrong, and we look forward to hearing from you throughout the site.