How to use Natvie sdk in xamarin.forms?

I'm developing mobile application using Xamarin.Forms for android and iOS. I have payment gateway(Braintree) SDK in native android(Java) and iOS(Objective c / Swift).
For android, I have tried creating binding library using *.Jar file, however not able to converted that Jar to dll. Facing errors like " does not implement interface member " even after creating the partial class also not working.
For iOS, I'm not able to find proper header file to convert.