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).
Segment your customers automatically with RFM
Segmenting your customers has always been touted as a powerful marketing tool but many stores avoid it because it can be time-consuming.
Repeat Customer Insights will automatically segment your entire customer base for you based on the valuable data Shopify has already collected for you. Ranging from 5 to 30 to over 125+ different segments using RFM and other models, you can pick how much power you want to harness.