In the quickstart tutorial we explain how to login.  After you login, you should notice some differences in the way the site appears; you're no longer an anonymous user, so depending on the permissions given to your account, you will be able to see different protected or hidden features of the site.

One difference is that (in most sites), the left navigation should be more extensive.  This is because you can now see most pages in the site, not just the ones that are published.  We'll go into that in more depth later.

You will also notice a new toolbar bar towards the top of the page with your login name at its far left.  It might include a link to your home folder, your preferences, undo, plone setup, and a button to logout.  The only one of those that requires no explanation is the logout button; you should hit that when you're done editing your site so no one sneaks some unexpected edits when you're not looking.  The others get their own page each in the Concepts part of the site; for now, they're not terribly important.