To recover your password and restore your Gmail account access, simply click on "I cannot access my account" under the login box, and you'll be sent to the password recovery page. You will be asked to type your email address into this bar, and as long as it's accepted as a valid Gmail account, it'll send you to a captcha image, which is one of those security images you sometimes see with slightly obscured, hard to read letters that you have to type correctly in the box provided. If you're having trouble reading off the letters, simply click the handicapped symbol next to the entry box, make sure your speakers are turned on, and you will be given an audio version of the captcha. Either way, once you've typed the numbers and/or letters given to you, Google will be able to verify that you're a real person, and not an automated program designed to compromise internet security. At this point, you'll be given your recovery options. You can have a password reset form sent to the alternate email address you provided when you created your account, which you can then open by logging into that email, or you can answer the security question you set up when you made the account. Once you've done one or the other, it will allow you to create an all-new password for your Gmail account.
As well, if you've forgotten your username, you can click on the forgot username link on the password recovery page to be sent to the username recovery page, where it will ask for the alternate email you provided when you sign up. Provide that, and a list of usernames will be emailed to your alternate account.