How can I get the email field value from a webform and use the rules module to send mail on submit event?
If you have an email field on the webform then that field will be available to you at webform/emails in the selector alongside the radio button = Component Value

That then becomes the 'to' field unless you then overwrite it.

You will also find 'select' type fields in that selector as you can then set email address for each of the options in the email settings directly.

You would use this if for example you had a field on the form for 'country' and needed to send the submission to different contacts based on which country was chosen.

Hope that helps