Xamarin Forms contain both iOS and Android Xamarin projects inside itself.
Initial setup for Android you can find here and for iOS here.
For functions which you want to use into Android and iOS part you must define into main Xamarin Forms project, for example:
public interface AppoxeeDependencyService
{
void setAlias(string alias);
void triggerInapp();
void enablePush();
}
Example project you can find at https://developers.mapp.com