Julie here from Shopify Support.
First off, it's great to hear that you're taking advantage of the 90-day trial. How are you enjoying it so far?
To check your remaining trial days, you'll just need to look towards the bottom of your admin dashboard. There, you'll see a dark indigo-coloured bar that will display the number of days you have remaining. You should also see the remaining number of days near the top of your admin Home screen. Here's an example of what this looks like:
Do you see something similar on your end? Because you signed up on April 19th, you should automatically receive the 90-day free trial. However, if you are seeing otherwise, feel free to respond here and let me know.
As you continue building your store, I recommend checking out our help doc on how to get the most out of your free trial. Here, you'll get an overview of Shopify's key features and learn how to familiarize yourself with the Shopify admin and your store settings. We also have a store launch checklist I think you'll find quite useful; this will provide you with a list of everything you'll need to complete before you can start selling and will help you ensure that your store is launch-ready.
I hope this helps! I'd love to hear more about your business. What do you plan on selling?