If you would like to use the Facebook tracking pixel, to help target your Facebook ads towards customers who have visited your site, you will just need to generate a pixel and then copy it over into your store's preferences.
To generate a Facebook tracking pixel:
Login to Facebook > Go to your Pixels tab in Events Manager > Click "Create a Pixel" > Click the "Create" button to finish creating your pixel.
Copy this pixel (it should just be a long number).
From here, you will need to add the pixel ID to your Shopify store.
Sign into your Shopify Admin > Online Store > Preferences.
Under the "Facebook Pixel" section, paste your pixel ID into the box, and save changes.
Need more help with this step? Check out Shopify's help docs for adding FB pixels by clicking here.