From the “New content” menu, choose a Mail form. In the “Configuration” field, you enter a form definition. The format for form definitions is defined in TSRef:

http://typo3.org/documentation/document-library/doc_core_tsref/FORM/

Here is an example form definition:

Often, the visitor will be asked to enter their email address, so support staff can get in touch with them. If the form contains an email address, a receipt can be sent to the visitor, in the form of a copy of the form data they submitted. To do that, enter the field name (from the form definition) which contains an email address in the “Field” field, and the email subject in the “Receipt subject” field.