You can start an online store by:
Using any of the ready to use ecommerce solutions like Shopify, Magento, Woocommerce.
Buying a SAAS based ecommerce service where you pay for the solution on monthly basis.
Getting a custom ecommerce solution developed by a company
Once you have an ecommerce solution, you will obviously need to set up the same using a domain name and server space.
Good and attractive website is important thing. There are many helpful tools for it creation, for example last time I used great form creator on https://aidaform.com/how-to-create-a-form.html quite helpful tool!