Insert data to the database from a form

from http://codex.ww.wp.xz.cn/Function_Reference/wpdb_Class#INSERT_rows

<?php
// if using a custom function, you need this
global $wpdb

$lastn = 'Last Name';
$firstn = "First Name';
$wpdb->insert( 'clients', array( 'last_name' => $lastn, 'first_name' => $firstn ), array( '%s', '%d' ) )

?>

Hi Ricardo, it is quite unfortunate the Codex does not contain a real example of form posting, storing, and retrieving to redisplay the form!!
I recently posted an outline of such as you wish to do, here:
http://ww.wp.xz.cn/support/topic/user-application-editing-form-store-retrieval-from-db

I think the step you are missing is the form must post to some existing page URL, such as the page itself! Therefore the php code in the page must recognize the incoming post prior to starting to display anything. See if you can make sense of my example.
You also have to deal with the question of user identity. How do you know who is submitting the form? My example asks for a WordPress login to do this. Hence the data can be retrieved by that specific user in the future, to be updated!

Rich