how do I mirror my iMac screen on my large screen tv using apple tv

Your iMac is likely too old unless it's a 2011 iMac or later. You are talking about the feature called AirPlay, the machines that support AirPlay are:

An Apple TV

• Apple TV (2nd generation or later) with software update v5.0 or later.
• AirPlay turned on in the Settings menu of the Apple TV.

A supported Mac

AirPlay Mirroring in OS X takes advantage of the hardware capabilities of recent Macs to deliver high frame rates while maintaining optimal system performance. The following Mac models support AirPlay Mirroring when using OS X Mountain Lion or later:

• iMac (Mid 2011 or newer)
• Mac mini (Mid 2011 or newer)
• MacBook Air (Mid 2011 or newer)
• MacBook Pro (Early 2011 or newer)
• Mac Pro (Late 2013 or newer)

Which Mac do I have?

• How to identify your MacBook Air
• How to identify MacBook Pro models
• How to identify iMac models
• How to identify Mac mini models

Well first the definition of the word BestBuy is incompetence, never go there for assistance. Use an Apple Store or bona fide AASP for assistance. No your blue ray player is of no value in this case.

Are you just trying to show movies or mirror your desktop period. if you just want to play movies there are third party apps that work. Beamer and Air Parrot work with Apple TV and SofaPlay will work with a smart TV.