First step of your integration will always be your store: the Zap needs to get notified whenever a new order is placed. First, search for your ecommerce platform in Zapier's actions database to see if there's an integration provided by your platform. If it is, use it, it'll make things easier.
If you can't find your platform among existing integrations, look for "webhooks by Zapier" and set your store to send webhooks to the address provided by Zapier. Let us know if you need help with that.