There are two ways to accomplish this.
Method 1: Client Settings Page
Click Client Settings on the Preferences submenu
Scroll down to the Email, Notification & Calendar Event Settings section.
Select an email template from the First Assignment Email menu. The email you select will be sent the first time a lead is assigned to one of your users. This method sends a user-specific email (can include a user's email address, signature, etc.) to your prospect immediately upon lead assignment.
Method 2: Manage Status Page
Click Manage Status on the Administration submenu
You can also send a general email by assigning an automatically-triggered email to your default status (usually "New"). Click the "Emails" link to the right of the New Status.
Select the Email you want to be automatically sent to new leads and click the Assign Button.
Set the trigger type to Auto or Both.
This will cause an email to be sent within a few minutes of the lead being added to your LeadManager.
Because this email could be sent before the lead is assigned to one of your users, the email you select should not contain any user-specific field tags. It should also be noted that if a lead is imported to a different status (as is possible with manual file imports) this email will not be sent.