Adding data to wordpress database using template - php

Question

I decided to create my first Wordpress plugin, then I switched to create a template instead, because I had problems with sending the data through form action. Now I have problems with adding data to databse using a template. I have a problem with my functions.php file I think. Can someone help me, please? The code:
<?php
if(isset($_POST['submitbtn']))
{
global $wpdb;
$data_array = array(
'OrderNum' => $_POST['OrderNum'],
'OrderStatus' => $_POST['OrderStatus'],
'ClientFirstName' => $_POST['ClientFirstName'],
'ClientLastName' => $_POST['ClientLastName']
);
$table_name = 'ordernumber';
$sql=$wpdb->insert($table_name, $data_array, $format=NULL);
if($sql == 1){
echo '<h1>Added</h1>';
}else{
echo 'Error';
}
}
?>
EDIT:
For some reason I can't add my whole second php file here so here is just the form, and I want to add I am a beginner in this all. `
<form method = "post">
<fieldset>
<p><label for="ClientFirstName">First name:</label>
<input type="text" id="ClientFirstName" name="ClientFirstName"/>
</p>
<p><label for="ClientLastName">Last name:</label>
<input type="text" id="ClientLastName" name="ClientLastName"/>
</p>
<p><label for="OrderNum">Order number: </label>
<input type="text" id="OrderNum" name="OrderNum"/>
</p>
<p><label for="OrderStatus">Order status: </label>
<select name="OrderStatus" id="OrderStatus">
<option value="Your order is wating for">Your order is wating</option>
<option value="Order is shipped">Order is shipped.</option>
<option value="Done">Done</option>
<option value="Error in order">Error in order </option>
</select>
</p>
</fieldset>
<p><input type="submit" value="Add order" name="submitbtn"></p>
</form>`