How to go about suing bestbuy?

I need some opinions on suing Best Buy. I went in for an advertisement back in January of 2016 for a buy one iPhone get another iphone free when you add a line but then realized a couple months later that that wasn't the case. I went in on an advertisement and the employee told me it had expired 2 days prior but they would honor it because they still had all the signs up for the promotion and he called someone over who introduced himself as a "manager" and said they would honor it. After that the alleged "manager" told the other employees to take down the signs so the same confusion didn't happen with another customer. Later on i found out that I had a balance to pay on this alleged "free" phone and when I called the store they told me there's nothing they can do about it that the employee's were wrong for doing that but that's all they could do for me but apologize for their employee's actions. What can I do, or how can I go about this?

Pull out the paperwork, including related ads if you have them.

Write a polite letter to the store explaining what you think the solution is, and asking them to do it.

I'm expecting you didn't read some fine print related to the "free" phone. You need to find something in writing that indicates that what occurred was incorrect.

You generally don't jump to suing unless you can prove that more cooperative solutions have failed.