Bagaimana cara mengatasi error seperti ini?

Warning: mysql_fetch_array() expects parameter 1 to be resource, boolean given in C:\xampp\htdocs\aplikasinilai\siniwa\proseslogin.php on line 25

No database selected

<?php
include ("conn.php");
date_default_timezone_set('Asia/Jakarta');

session_start();

$username = $_POST['username'];
$password = $_POST['password'];

$username = mysql_real_escape_string($username);
$password = mysql_real_escape_string($password);

if (empty($username) && empty($password)) {
    echo "username dan password masih kosong";
    break;
} else if (empty($username)) {
    header('location:index.php?error=2');
    break;
} else if (empty($password)) {
    header('location:index.php?error=3');
    break;
}

$q = mysql_query("select * from user where username='$username' and password='$password'");
$row = mysql_fetch_array ($q);

if (mysql_num_rows($q) == 1) {
    $_SESSION['username'] = $username;
    $_SESSION['fullname'] = $row['fullname'];
    $_SESSION['gambar'] = $row['gambar'];

    header('location:admin/index.php');
} else {
    echo "username atau password yang anda masukan salah";
}
?>