What's the best way to start an online store?

It depends really in what you are selling to who and how they pay.

In most cases Shopify or Woocomerce are the go to tool. Othertimes Magento, Snipcart or an other tool could be a good fit.

Even a custom solutions is sometimes the best choice (something I did for my ecommerce business, after trying Woocomemmerce, Magento and Snipcart first).

I think it depends on what you're selling. If it's physical goods, you can get up and running quickly with Shopify which is a complete one-stop-shop option, but it might not always be the most economical option. If it's digital products, you could look into something like Gumroad. If you don't need the robust back-end that Shopify provides, then you can spin up a store super quick using just your phone, with https://onuniverse.com/.

For most use cases - Shopify

If you don't wanna pay Shopify's transaction fees - Wix
If you're a designer and wanna show off - Webflow Ecommerce
Fast to launch, ease of use at the cost of fewer customization options - Squarespace

For more defined use-cases - Weblium, Ecwid, Volusion, Bigcommerce (large volume), Magento (enterprise-grade), and so on

If you're a developer - WordPress/WooCommerce combo (low-code)

Edit: If you'd like to proceed with Shopify, then let me know I offer a free store building service. You can also visit my site to learn more.

Depends on the store. There are a ton of interesting solutions coming out.