To add or edit content on a Drupal site, usually you have to first be registered as a user. (Sometimes the site administrator has chosen to enable ‘anonymous’ posts of things like comments, in which case you can post them without registering.)
In some cases, a site administrator will add you as a user. If so, they will send you a username and password that you can use to log on.
Otherwise, your site may be configured to provide a small form called ‘User login’ on the main page. Click the link that says ‘Create new account’:
NOTE: If this ‘User login’ form is not visible on your website, you can visit http://www.example.com/user to find a page with a similar form.
The next page that comes up may have some information on the site's policies for registration. To register, enter a username of your choice and your email address and hit ‘Create new account’. Within a few minutes, you should get an automatically-generated email confirming your registration and giving you an initial password to use. Now you're ready to log in. Here is the default ‘Create new account’ form: