Mobile app that communicates with Garmin watches and retrieves health data

Hello, 

I want to develop mobile application (probably in JavaScript) which will retrieve data from Garmin watches. I would like to get some health snapshots in real time and get the health data into the mobile application. What would you recommend?

Should I use API, Health SDK or Fit SDK?

I do not want to develop app for watches, just for the mobile device (Android/iOS).

Is it possible to get accelerometer data into your mobile phone?

I am just starting with Garmin, so I appreciate every hint.

Thanks a lot :)