Hi I got a Free Subscription for 3 months from best buy ? How Can i use it ?

Best Buy should email redeem instructions after checkout. Follow that, and you should be set to go.

First you’ll need a personal Apple ID account, with all the user information that goes with that, including a verified email address (respond to test message), and a valid payment method (as required for starting any subscription).

Second, you’ need a supported device on which to view Apple TV+ content.

How to create a new Apple ID - Apple Support (if you don’t have one yet)

List of Apple TV app-supported devices - Apple

Get the Apple TV app or Apple TV+ app on your smart TV, streaming device, or game console - Apple Support

All the ways to watch Apple TV+ - Apple Support

Change, add, or remove Apple ID payment methods - Apple Support

Note that the terms and condition say the the offer is for new or returning subscribers. That excludes existing subscribers.