Check you subscription details. The confirmation email should specify the duration of the trial and the first day of payment. Also see View, change, or cancel your subscriptions - Apple Support.
Check your purchase history for anything that is billed to your Apple ID, including subscription payments. See your purchase history in the App Store or iTunes Store - Apple Support.
If you see 'itunes.com/bill' or an unfamiliar charge on your statement - Apple Support
Identify legitimate emails from the App Store or iTunes Store - Apple Support
Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support