Esempio di base.
Codice PHP:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<script language="JavaScript" type="text/JavaScript">
function validateForm()
{

    if (document.forms[0].elements[0].value == "")
    {
        alert ("Il campo frmInput è vuoto");
        return false;
    }
    return true;
}
</script>
</head>
<body>
<?php 
if(isset($_POST['frmInput']))
{
    if(empty($_POST['frmInput']))
    {
        echo "Il campo frmInput è vuoto";
    }
    else
    {
        $value = $_POST['frmInput'];
        //INSERISCI $value nel DB
    }
}
?>
<form onSubmit="return validateForm();" action="<?php echo $_SERVER['PHP_SELF'];?>" method="post" name="frm">
<input name="frmInput" type="text" value="">
<input name="frmSubmit" type="submit" value="Schizza !">
</form>
</body>
</html>



Per le cose che non conosci consulta la guida on line di php