This should be staightforward, possibly easier than finding this help page!
Look on the right of the website, there will be a panel labelled "User Login", with fields for your name and password. To create a new account click the link promising to do that for you, fill in your details, demonstrating you're not a malevolent canned-meat robot by passing the word verification test, and click "Create new account". et voila.
However, some robots can apparently pass this test (or perhaps more accurately, we have humans attempting to spam our site), so your account needs to be activated by an administrator before you can use it. To help avoid looking like a spammer you can fill out some of the personal information fields with something sensible, rather than saying you're a Oil Magnate from Tanzania. (Spammers, don't bother trying this since we'll spot your posts and kick you off anyway.)
Activating your account may take a few hours, but you will receive an email notification when it is done. If find you've waited more than a day or two, possibly you've been missed, or perhaps your email address just looks a bit suspicious. Email firstname.lastname@example.org and point out that you're not a spammer and it should be resolved quickly.
Once activated, you have access to more of the facilities of the website, including posting comments, to the forum, and access to certain information which isn't revealed to all and sundry.
Note, if you have an account already, please don't create a new one, just click the other link "I forgot my password" link. This will send you an email which will get you onto the site long enough to reset the password to something you can remember.
A final point: if you're joining because you're on the committe or otherwise in some official capacity, please read this other help article.