I'm Miles from the Social Care team at Shopify.
While on a trial, if you select a plan and add your payment details, there will be no charges on the account until your trial is finished. You'll still be able to access the great features of the plan you select for free until the time you are charged.
If you've already selected a plan at yearly, you can downgrade it for free before the end of your trial by heading to Settings > Plan and Permissions and clicking "Compare Plans". Please note that when you downgrade from annual subscriptions to monthly, you may lose some features such as carrier calculated shipping. This can be added on again for $20 USD/month on monthly plans, by subscribing to the yearly plan again or upgrading to the Advanced Monthly Plan.
If you'd like to get an idea around pricing, check out our pricing guides.
I hope your trial period is going well. How are you finding Shopify so far? We've got some great tips on getting set up with Shopify that you can find here as well.
Have a great day!