I am doing dropshipping, how to create an automation to let my suppliers when the product is order, so they can fulfill the order?

You could achieve this by creating an automation to send order/customer info via email to the suppliers/vendors when the products from them are ordered. Here is a step by step guide below,

1. Create a new rule by clicking the "Create New Automation" button in the App
2. Define the conditions
1. Select the 1st condition from the dropdown "Product is ordered"
2. Click the "Add Condition" button
3. If you are using vendor to differentiate the product suppliers, select the "Product is from Vendor", and then input the vendor name (the vendor name should same to the name in your Shopify admin)
4. If you want, you can keep adding conditions by clicking the "Add Condition" button and configure the conditions.
3. Define the actions
1. Select the 1st action from the dropdown "send email notification"
2. Defined the recipients and subject shown in the screenshot below
3. Input the message you want in the email body
4. Select the proper data table template from the dropdown, in this case, we want to include ordered product info and shipping info
5. We can also add a tag to this order to indicate the supplier email has been sent, so click the "Add Action" button
6. Select the "tag order with", and input the tag you want to add.
4. Save the setup, and you could see the automation is shown up in the automation list.
5. Click the "see logs", you will see the details of each run

You could combine multiple conditions and multiple actions to meet your needs. If you could not find combination, simply email us. We are delighted to learn your demand and improve our App.