The log in button is at the top of the site. Click the log in button and type your username or email in the field titled "Username" and type your password in the field titled "Password". (Remember, usernames and passwords are case and space sensitive). Then hit the "Log in" button to the bottom of these fields.
HINT: Checking the "Remember me" box will keep you logged in so when you return to the site, you won't have to do it all over again.