Go to your web site and look for the log-in block/link in the left-hand side block. Depending on the theme of your site and how it is customized, it may look different or be located elsewhere (such as in the top right corner). It should look similar to this:
If your site does not allow people to create user accounts, the log in box may be hidden. In that case, go to www.yourdomainhere.com/user. The /user page functions as the log in page when you aren't logged in and your account page when you are logged into the site.
You can tell when you're logged in because a menu (the navigation menu) should show up that has the create content link, administration menu (if you have access to the admin area), etc. The title of the menu will be your user name.
Your login status is "remembered" on your computer, so when you return to the site, you may still be logged in. To log out, click on the"log out" entry at the bottom of the navigation menu.