The form you currently have should probably be set as a method="post" instead of the default get. The action is optional, but will post back to this form.
From the HTML to PHP world, the HTML form is filled out on the client and the submit sends it back to the provided action or to the originating page if action is not specified. It will include the fields: name, email, Tel_no, and subject. Note the change in Tel_no, you cannot have a . in your name, which is converted (as an external variable) to _. For more information on this, refer to: http://php.ca/manual/en/language.variables.external.php
Now, the email itself is done with the mail() command. Let me find a couple of tutorials.
That one is good, but doesn't provide any type of security.
How to prevent email injections
And a bit on general mail security.