Thanks for taking the time to register here at MetaFilter. After registering you'll be able to post both links and comments, as well as customize the Metafilter interface to your liking. The site depends on the great contributions of people like you.
But first, some general principles and guidelines that help guide the site and the community.
One of the guiding principles here is trust in each other. The index page of metafilter.com can be added to by almost everybody that has signed up, as there is very little editing or deleting going on. Anyone can post a comment in a thread, and say whatever they feel.
I give you the ability to do this because I trust you. I trust that you'll act in a civilized manner, that you'll treat others with opposing viewpoints with absolute respect and that you'll contribute in a positive way to the intelligent discussions that take place here every day.
I give you the benefit of the doubt, because I trust you, so all that I ask is for you to honor that trust and promise to become a good contributor.
When the level of discourse here is high, conversations are typified by good contributions from a variety of viewpoints and experiences. Be respectful of others, bring your experiences and share them with everyone. If you make a statement of fact, show supporting evidence (hopefully as hyperlinks to other web resources). When everyone brings intelligent, thoughtful commentary to a thread, MetaFilter is the best it can possibly be. If you have a unique perspective on a topic, by all means contribute. If you'd like to express an opposing viewpoint in a respectful way, by all means contribute. If you'd like to add supporting facts and statistics or stories from others you've found elsewhere on the web, by all means contribute.
One of the things that makes MetaFilter valuable to its members is the contributions of the community. This means that a comment is appreciated for the value that it adds to the discussion. With many topics, members go into a conversation already having a general idea of how they feel about the subject. The most valuable posts present new information, new perspectives, and new ideas that challenge each of us to consider not just what we believe, but why we believe it.
For more information, there's a lengthy About section, and Guidelines for links and comments. If you have more questions or want to talk about the site itself, take a look at the entire MetaTalk subsection of the site for a variety of topics.
Due to a few opportunistic members that ignored guidelines in the past, there is a one-week waiting period after signup, before you are allowed to post a new question to Ask MetaFilter or a new post to MetaFilter. You can post comments to MetaFilter and MetaTalk, and answers to Ask MetaFilter right away though.
Due to the bursting size of the community, its use of resources, and the cost of running the servers, all new users have a one-time $5 charge, to help defray these costs. If you sign up an account to pimp your product, act like an ass, or generally just do things that break the guidelines you will be booted and there will be no refunds.
Still want in? Go ahead and sign up for an account here. At the end of the process you'll be directed towards Paypal, and upon your return, you should be able to login and post away.