A free trial becomes a paid subscription on its end date, unless you cancel before then. Cancelling does not cut the trial short: your access stays open until that date.
Before you start, read the end date of the trial and the price that will apply afterwards.
Start the trial, then set an alarm on your phone right away, two days before that date.
If the app does not suit you, cancel before the date: Settings > your name > Subscriptions (iPhone), or Google Play > profile picture > Payments and subscriptions > Subscriptions (Android). You should see the date your access ends.
If it does suit you, do nothing: the subscription starts on its own at the end date.
If you don’t see it: the trial may have been taken out on the publisher’s website rather than in the app store. Sign in on that site, then look for « Account » or « Billing ».
Limitation: a trial taken out outside the app store cannot be cancelled in your phone settings. The confirmation email you received at the start says where to manage it.
To picture it: a free trial works like a free first month at the gym: if you do not cancel before the set date, the payments begin automatically.