There are several different methods of registration, each with a different message that can be changed.

You need to edit the text found in /Themes/default/languages/Login.english.php.

Search that file for:

• $txt['register_immediate_message'] - Immediate registration email (without activation)
• $txt['register_activate_message'] - Activation email
• $txt['register_pending_message'] - Admin approval registration

depending on the email you want to modify and change their contents with the text you would like to use.

You need to edit the text found in /Themes/default/languages/EmailTemplates.english.php.

Search that file for:

• 'register_immediate' - Immediate registration email (without activation)
• 'register_activate' - Activation email
• 'register_pending' - Admin approval registration
• other possible registration email texts are: 'register_openid_activate', 'register_coppa', 'register_openid_coppa', 'register_openid_immediate', 'register_openid_pending'.

The structure of each message is as follow:

'register_activate' => array(
		/*
			@additional_params: register_activate
				REALNAME: The display name for the member receiving the email.
				USERNAME: The user name for the member receiving the email.
				PASSWORD: The password for the member.
				ACTIVATIONLINK:  The url link to reactivate the member's account.
				ACTIVATIONLINKWITHOUTCODE: The url to the page where the activation code can be entered.
				ACTIVATIONCODE:  The code needed to reactivate the member's account.
				FORGOTPASSWORDLINK: The url to the "forgot password" page.
			@description:
		*/
		'subject' => 'Welcome to {FORUMNAME}',
		'body' => 'Thank you for registering at {FORUMNAME}. Your username is {USERNAME}. If you forget your password, you can reset it by visiting {FORGOTPASSWORDLINK}.

Before you can login, you first need to activate your account. To do so, please follow this link:

{ACTIVATIONLINK}

Should you have any problems with activation, please visit {ACTIVATIONLINKWITHOUTCODE} use the code "{ACTIVATIONCODE}".

{REGARDS}',
	),

Changing the text associated with 'subject' results in altering the subject of the email, while changing the text associated with 'body' will alter the actual message. Additional parameters (as listed in a comment in the file at the beginning of each message) can be used to introduce personalized information for each email (such as the member's username or realname).