Turn on and off ADS and SEO

Hi guys,
I'm trying to build a page that would rank on SERP. However, I have a turn on and off ads feature, which I incorporated using PHP and Javascript, and it looks something like this:
http://www.domain.com
http://www.domain.com?ads=on

How to tell Google not to index or crawl the second page?

I don't know the exact formula but you can do this with robots.txt, or .htaccess ... actually, I think you can also do it with SEO plugins, I'm pretty sure there are plugins (for Wordpress I mean) that can noindex URLs

try to ad this in your robots.txt
User-agent: *
Disallow: ?ads=on