I received an email from 'Apple"saying I had purchased a subscription to "Drop Box" free trial and then after a month I will be charged $13.99 per month...is this a scam ??

Check the email (e.g. does it contain your billing address from your iTunes account, does it have spelling or grammar errors) :

Phishing and similar emails : Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support

Genuine iTunes emails : Identifying legitimate emails from the iTunes Store - Apple Support

Viewing an iTunes account's purchase history : See your purchase history in the iTunes Store - Apple Support

Viewing/managing subscriptions on an account : View, change, or cancel your subscriptions - Apple Support

Apple will always address you by your name or the name they have on file for you, not Dear Customer, Dear Client or by using your e-mail address. The e-mail will be from @apple.com or @iTunes.com. E-mail addresses can be spoofed. You can go to Mail/View/Message/Show all Headers to see more. Apple e-mails will never contain an attachment. Apple will never request personal information such as Social Security numbers.

The only exception to the above I have noticed is if you order something from the Apple Store (apple.com), your receipt will be addressed to Dear Apple Customer. That is a receipt for a purchase you initiated.

Send the e-mail to Apple as an attachment to a new e-mail before deleting it. You can forward as an attachment by going to Mail/Message/Forward as attachment.

reportphishing@apple.com