我对注册系统编码的错误尝试

所以我试图建立一个登录系统的网站开发，我是很新的php，所以我已经跳在深端有点。我的目标是创建一个注册表单，如果密码不匹配，它不添加数据，并显示一个错误，如果用户名采取。我已经找到了这两个代码，但我不确定如何实现它们在一起。

<?php
$dbhost = "localhost";
$dbuser = "root";
$dbpass = "";
$dbname = "immo";
//Connection to the DB
$con = mysqli_connect($dbhost,$dbuser,$dbpass,$dbname) or die ($dberror1);
    session_start();
    if (isSet($_POST['Register']) && isSet($_POST['Username']) && isSet($_POST['Password']) && $_POST['Username'] != '' && $_POST['Password'] != '') {
        $Password = $_POST['Password'];
        $passMD5 = md5($Password);
        $Username = $_POST['Username'];
        $q = mysqli_query($con, "SELECT * FROM `users` WHERE `Username`='$Username'");
        if (mysqli_num_rows($q) > 0) {
            echo 'That username is already taken.';
        }
        if ($_POST['Password']!= $_POST['ConfirmPassword']){
            echo("Oops! Password did not match! Try again. ");
        }
        else{
            $qq = mysqli_query($con, "INSERT INTO `users` VALUES ('', '$Username', '$passMD5')");
            if ($qq) {
                echo 'Registered successfully!';
            }else
                echo 'Failed to register.';
        }
    }
?>

<form method="post" class="registration_form">
  <fieldset>
<legend>Registration Form </legend>
<table>
 <tbody>
<tr>
  <td class="label"><label for="Username">Username :</label></td>
  <td class="input"><input type="text" id="Username" name="Username" required/></td>
  <td class="error"></td>
</tr>
<tr>
  <td class="label"><label for="Fname">First Name :</label></td>
  <td class="input"><input type="text" id="Firstname" name="Firstname" required/></td>
  <td class="error"></td>
</tr>
<tr>
  <td class="label"> <label for="Lname">Last Name :</label></td>
  <td class="input"><input type="text" id="Lastname" name="Lastname" required/></td>
  <td class="error"></td>
</tr>
<tr>
  <td class="label"><label for="e-mail">E-mail :</label></td>
  <td class="input"><input type="email" id="Email" name="Email" required/></td>
  <td class="error"></td>
</tr>
<tr>
  <td class="label"><label for="Password">Password:</label></td>
  <td class="input"><input type="password" id="Password" name="Password" required/></td>
  <td class="error"></td>
</tr>
<tr>
  <td class="label"><label for="ConfirmPassword">Confirm Password:</label></td>
 <td class="input"><input type="password" id="ConfirmPassword" name="ConfirmPassword" required/></td>
  <td class="error"></td>
</tr>
<tr>
  <td><input type="hidden" name="formsubmitted" value="TRUE" /></td>
  <td><input type="submit"  id="Register" value="Register" name="Register" /></td>
  <td class="error"></td>
</tr>
 </tbody>
</table>
  </fieldset>
</form>