Before you can post, you’ll have to register for an account, and wait for the confirmation from an administration:

Register for an account

You can also find the register link in the sidebar of every page, to the right:

We manually review every new account requests since there is quite a bit of spam on this site. It might take a few hours or days before we accept or refuse your account. Once your account is active, you’ll be able to submit posts and comments directly.

Once you have an account, there is extensive documentation on how to use the blog system at: