need help locating wp-config.php file, and code to add another administrator to my wpmu account

1) Not being a techie I don't know where to find the wp-config.php file that I need to modify the code to add a second administrator to my wpmu account.

2) I also don't know exactly what to do to the code to add the second administrator.

Ultimately, I need to add my office assistant, Linda, to the list of administrators since she does most of my website updating.