adding function.php

Why do you need it?

wp-content > themes > your_theme > functions.php is the default location of functions.php in any theme. If if is not there, just open a text editor and write:

<?php
// Functions will go here, but this is just a comment

?>

and then save the page as functions.php. Now write or copy-paste unlimited functions you desire. But always write the functions within the php tags.