Our 90 day free trial is automatically applied to all new accounts.
If you have not yet selected a plan, you will see a banner at the bottom of your screen inside your admin indicating how many days you have left of your trial.
If you have selected a plan, you can see the date you will be charged for your subscription fee under Settings > Plan and permissions > Compare plans > Change your billing period. You can also see all upcoming charges under Settings > Billing!
Please keep in mind that you will not be charged for the plan you've selected until your 90 day trial has ended. If you start selling while you're on your trial, you will be charged transaction and shipping label fees if you incur these charges.
Where you're just getting started, I'd highly recommend working through our "Getting Started with Shopify" course here.
Hope this helps! Our support is available 24/7 if you have any questions!