Logging In
If you look at the right hand part of your Plone website, near the top, you will see a log in hyperlink:
This is how you log into the content management side of the site. Click log in, and a form will appear, looking like the following:
I have marked in red the fields where you must enter in your login id and password, which both should've been set when your account was created.
Before hitting the log in button, take notice that this log in screen defaults with several other hyper-links on it, including a way for people to register for a site userid and password, and a method to retrieve a forgotten password.