If you're the administrator, you can log in with your ID and password and begin administering the site.
Where to log in
Depending on your site setup, your front page, or another page on the site, may show a login block like this:
Whether or not you see that, though, you can always log in by going to this page:
<your site domain>/user
or, depending on your site setup,
<your site domain>/?q=user
Provide the ID and password, and you're in!
If you've forgotten your password, hit the "Request new password" link. You'll receive a new password by email, at the address specified when your (or another administrator) set up your account.