Unfortunately, you can't just use it anywhere you like. Here are some of the terms and conditions for use of the new font.
Subject to the terms of this License, you may use the Apple Font solely for designing or developing applications for Apple-branded products running on Apple’s iOS or OS X operating systems, as applicable. The foregoing right includes the right to show the Apple Font in screen shots, images or mock-ups of such applications for use on Apple-branded products running iOS or OS X only.
You may use this Apple Font only for the purposes described in this License and only if you are a registered Apple Developer and an eligible paid Apple Developer Program member, or as otherwise expressly permitted by Apple in writing.
Hit the link below to download the font and learn more about Apple's TextKit and Font Specifications.
Read More [via AppleInsider]