I have a strange bill! I don’t bought nothing for this amount of money!How to know what is it 39.99$ ?

Start reading this support page:--> Subscriptions and Billing – Official Apple Support, and check each of the first 4 sections.

If you cannot find the information you were looking for, please contact the iTunes Store Support Team directly, using this link:--> iTunes Store & App Store – Get support for various issues, and choosing: your country, then Apple ID, then iTunes & App Store. If your problem is not among the options shown there, tap/click on the option “the topic is not listed” and describe your issue.

Regards

Giulio