Meta Tag Validation in Wordpress

One of the affiliate partnerships I accompany is asking me to put a meta tag in the html code of my index page. The thing is I am on Wordpress and the index.php file is just a few lines long. Someone can tell me how to find the index html page (I can see it using 'View Source' in my browser) and modify it?
Thanks!

The HTML Code that you see by clicking "View Source" in your browser is the result if the PHP code, so you won't find just the HTML page.
Anyway, search on google the plugin "Meta Tag Manager" and install it, you should be able to do what you want whit this.

Thanks very much! Solved!