You can find it in the php manual if you do a little search
Here is that code adjusted to what you need:
[php]
<?php

/// here is your database connection

/// than the query
$result = mysql_query ("select field_1, field_2, field_3 from table") or die (mysql_error());

$filename = $_SERVER['DOCUMENT_ROOT'].'/test.xls'; //// or whatever.txt

if (!$handle = fopen($filename, 'a')) {
print "Cannot open file ($filename)";
exit;
}
// Let's make sure the file exists and is writable first.
if (is_writable($filename)) {

while ($row = mysql_fetch_row($result)) {
$row_content = implode ("\t",$row)."\r\n";
// Write $row_content to our opened file.
if (!fwrite($handle, $row_content)) {
print "Cannot write to file ($filename)";
exit;
}
}

print "Success, content wrote to file ($filename)";

fclose($handle);

} else {
print "The file $filename is not writable";
}

?>

[/php]

Try to do the basics first (like reading the manual)