How to avoid SQL injection in this Php code and create MYSQL queues? [duplicate]

I have this code in a system that I am developing, it works but I know that I have to put security in it and also leave it to work in the long term, for which I would like to know how to prevent the database from falling down, to several users enter information at the same time, finally there is a problem if I do not divide the entire code into classes?

<?php
if(isset($_POST["submit"])){
include_once 'resource/Database.php';


try {


$sql = "INSERT INTO capturar_pedido (cliente,
orden_de_compra,
producto,
unidad,
cantidad,
fecha_de_embarque,
notas,
etiquetado)
VALUES ('".$_POST["cliente"]."','".$_POST["orden_de_compra"]."','".$_POST["producto"]."','".$_POST["unidad"]."','".$_POST["cantidad"]."','".$_POST["fecha_de_embarque"]."','".$_POST["notas"]."','".$_POST["etiquetado"]."')";


if ($db->query($sql)) {
     echo "<script type= 'text/javascript'>alert('New Record Inserted Successfully');</script>";
} 
else{
     echo "<script type= 'text/javascript'>alert('Data not successfully Inserted.');</script>";
}

    $db = null;
    }
catch(PDOException $e)
    {
    echo $e->getMessage();
    }

}

?>

<?php
include_once 'resource/Database.php';

$sql = $db->prepare("SELECT * FROM capturar_pedido");
$sql->setFetchMode(PDO::FETCH_ASSOC);
$sql->execute();


if($sql->rowCount() != 0) {

?>
<table class="table table-condensed">
   <tr>
      <td>cliente</td>
      <td>Orden de compra</td>
      <td>Producto</td>
             <td>Unidad</td>
             <td>Cantidad</td>
      <td>Fecha de embarque</td>
      <td>Notas</td>
      <td>Etiquetado</td>


   </tr>
 <?php     
 while($row=$sql->fetch()) 
 {
      echo "<tr>".
           "<td>".$row["cliente"]."</td>".
           "<td>".$row["orden_de_compra"]."</td>".
           "<td>".$row["producto"]."</td>".
                     "<td>".$row["unidad"]."</td>".
           "<td>".$row["cantidad"]."</td>".
                     "<td>".$row["fecha_de_embarque"]."</td>".
           "<td>".$row["notas"]."</td>".
           "<td>".$row["etiquetado"]."</td>".


           "</tr>";
 }

}
else
{
     echo "don't exist records for list on the table";
}

?>
</table>