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:

In the “Jump to page” field, you enter which pages the visitor should be redirected to in turn after the form data has been submitted the first time. You would typically redirect the visitor to a page displaying a preview of the submitted data and finally a page saying “Thank you for your message” or a similar message. If you do not select any pages to redirect to, the form data will be submitted via email right away, without being previewed. When the visitor has been redirected to the final page, the form data is submitted  to the email address in the “Recipient-email” field.

To display a preview on a page, like in the above screenshot, you insert a Mailform preview plugin. This plugin is not available from the “Form elements” menu, but can be inserted as a “General plugin”.