Prova così:

Codice PHP:
<?php
    error_reporting
(E_ERROR);
    
$connect=mysql_connect("localhost","root","");
    
mysql_select_db("forum",$connect);
    
$query="SELECT* FROM utenti WHERE Username='".$_POST['Username']."'";
    
$ris=mysql_query($query);
    if(
mysql_num_rows($ris)>0){
        echo 
"Username gia in uso";
        }
        else{
            if(
strcmp($_POST['Password'],$_POST['Conferma_Password'])!=0){
            echo 
"Le due Password Coincidono";
            }
            else{
            }
        }

?>