Sorry buddy..these sitewide Best Buy coupons don't exist. I do see 20% off coupons often though, but they work on small appliances only. Over Black Friday and Cyber Monday I see 10% off coupons for the outlet. Their hottest deals section is awesome so if you see a good price there, it's better than any old 10% coupon.
Take it to the store with you and hand it to the cashier when you pay for your products. Now if you mean how do you get one, well those are for My Best Buy members. Receiving a coupon is not guaranteed though. Make sure in your preferences in your My Best Buy membership that you opt IN for email.
It's pretty cool! They don't only take smartphones and tablets. You can trade in gaming consoles, laptops, Macbooks, smartwatches, and video games, too!
I traded in an iPhone 6 Plus 16GB and got $120 back. You can use the gift card on any purchase at Best Buy, like cash! It was super easy and convenient, however expect less than the value you'd get selling it yourself on eBay or Craigslist.
They have a cool tool that tells you what you can trade in and what you'd get for it here: https://tradein.bestbuy.com/client/#/catalog/.