Shopify provides a free trial and upon shop creation, you can create your shop with Shopify subdomain URL.
For example, *.myshopify.com (Here, * means available name).
Now, let’s come to your question, which is – Do I need a domain name for shopify?
Shopify provides a free subdomain URL to create your online store. Now, if you go with *.myshopify.com sub domain URL, then buying a domain is not a requirement.
For creating a store brand, online identity, building trust and unique memorable domain URL, you need to buy a domain name when you launch your online store.
Can I use my own domain name to Shopify?
Yes, you can buy a new domain name from Shopify or you can transfer/connect your existing domain name and use it in Shopify.