When you start working on your repeat customers, first figure out your ratio of new-to-repeat customers.
As the example above says, above a 30% ratio is healthy. That means out of ten customers, three will be repeat customers.
If you're under 30%, much of your beginning work will be on improving that. There are a lot of tactics you can use: better branding, email reminders, New Customer Welcome campaigns, loyalty programs, etc.
Honestly, test them all and see what works.
(Just not all at once)
Your specific merchandise and business model will determine which tactics work well for you. Borrow ideas from other stores and even other industries entirely. I take a lot of ideas from ecommerce and adapt them into my app and I used a lot of concepts from the manufacturing industry in my consulting company.
Even if your ratio is healthy, that doesn't mean you can ignore it. You'll want to continue improving your repeat customer relationships to keep the ratio healthy and grow other aspects of your customer's lifecycle (e.g. Lifetime Value optimizations).
Use cohorts to find out who the best customers are in your Shopify store
Repeat Customer Insights will automatically group your customers into cohorts based on when they first purchased. This will let you see how the date customers bought would impact their behavior.