You will need to buy a TB3 eGPU enclosure. These range anywhere from $200 to $500 and typically come without a GPU. There are some more expensive options (like the Aorus Gaming Box with a GTX 1070/1080 or RX 580) that already come with a GPU installed.

Here is a nice comparison of the eGPU enclosures available today: https://egpu.io/external-gpu-buyers-guide-2018/

My personal recommendations are the Akitio Node Standard/Pro or Sonnet Breakaway Box 350/550/650. These companies are reputable companies in the Thunderbolt hardware realm.

Note that I am not affiliated with any of the companies mentioned above.

You'd have to buy an eGPU case, along with a GPU. Apple recommends Sonnet case with AMD GPUs [0].

However, as seen here [1], people have managed to use other GPU cases, with NVidia cards.

0: https://support.apple.com/en-us/HT208544

1: https://egpu.io/build-guides/

As others mentioned, you will need a case. You will, however, also need a device with TB3.

You would need an external Thunderbolt 3 enclosure for your GPU