Step 1: Create a password protected landing page for email capture only. You can do this using Shopify.
Dashboard > Themes > Customize > Password Page
This password page must include the email capture plugin with "coming soon" as the heading.
Step 2: To encourage potential customers in signing up for emails once you launch, you can offer an exclusive coupon on launch day in exchange for their email.
Step 3: Follow through with that offer. On the day of your launch or the night before, send out an email with the exclusive coupon code. Inform those who receive this exclusive coupon code that it will expire in X number of hours. If you give them a limited time frame to purchase, they will be more likely to take advantage of the offer.