Congratulations on launching your store! Were you hoping to just dropship from one specific brand? Most of the dropshipping apps available here should allow you the ability to filter out your search results by vendor. This way, you can view products from that specific supplier only and just import those products.
If there is a specific supplier that you're interested in sourcing from, who is not integrated with Shopify via a dropshipping app, then you can also just create the products on Shopify and fulfill them externally. In that case, when you make a sale you would place your order through your supplier on their external website, and mark the order as fulfilled in Shopify.
If you were working with a supplier externally, then you also have the option to list the supplier as a custom fulfillment service. This allows you to send your supplier your orders by email automatically, when fulfilling them in Shopify. This requires that the supplier can process orders via email, so you'd want to contact them directly to ask if this would be an acceptable way to work with them. I hope this information is helpful and if you have any other questions I'm happy to help!