From the homepage of your site, click Log In. If your site doesn't have the option to login, just add the words "login" after your site's URL (www.mysiteurl.com/login).
On the login screen, enter your Email Address and Password.
Check Remember Me if you'd like to stay logged in on your next visit. Don't check this if you're on a public computer.
Forgotten passwords can be recovered by clicking Forgot Password. You'll just need the original email address that was used when the account was created.