My name is Aaron, I'm a guru here at Shopify!
No problem! For a buy one get one function like this, your best bet is to use an app. Here's a couple of our more popular apps for bringing in that feature:
If none of those are quite what you're looking for, there's lots more available in our app store!
Hope that's helped! Please feel free to give us a call or start a live chat at any time, we're open 24/7 for your convenience and always happy to assist!
Aaron | Shopify Guru