Your Apple ID and password is the key to gaining access to the App Store, and Apple has a username and password locator tool to recover forgotten information. Your Apple ID is usually the same as the primary e-mail address attached to the account, and cannot be deleted once created. As a security precaution, Apple resets your account password and allows you to create a new one rather than display the previoius password.
Visit the Apple Forgot ID page, and click "Forgot Your Apple ID?" underneath the text field.
Enter your first and last name, current e-mail address and up to three previous e-mail addresses. Click "Next" to use the Apple ID locator to pull up all Apple accounts linked to the entered e-mail addressees.
Click "Email authentication" and click "Next" to have the Apple ID sent to your e-mail address. Open your e-mail account, and open the message from Apple. The message contains a link that redirects you to the password reset screen.
Enter the new password, and re-enter it in the confirmation box. Click "Reset Password" to submit the changes. A confirmation e-mail is sent to your e-mail account with the name of your Apple ID and a confirmation of the new password.
Click "App Store" on your Macintosh's desktop dock and click "Store" on the application's menu. Click "Sign In" and enter your Apple ID and password. Click "Sign In" to log into your account.