Go to your Ads Manager and click to open the header menu. Under Assets, look for Pixels:

Press “Create a Pixel” button. Name your Pixel as you like and Facebook will automatically generate it.

After creating it in the next step you will see Pixel code. Just press “View Pixel Code” button. All you need is a pixel ID number, which is highlighted. Copy only that number. Go to your Shopify store Admin page and paste it into Facebook Pixel section, which you can find in your Shopify account under Online Store > Preferences.

Credit: Oberlo Blog