Open the Trezor's box. After breaking the seal, if it is hard to open it this is a good sign that the device has not been handled or compromised.
You will find inside the Trezor One device, a USB cable to connect it to the computer, a Getting Started card, some stickers, and 2 cards to write down the seed.
Trezor's Getting Started page
Choose the right Operating System and click Get desktop app to download it.