As it turns out, this is really what I needed:
Just like Patrick said. On the ending, I went to the Sandcastle's "Picking" demo and followed the "Show Cartographic Position on Mouse Over" method to convert the X and Y coordinates (based on canvas) into Cartesian3 (long, lat, height).
Here is for those who may need it in future.
var myPosition = billboard.computeScreenSpacePosition(scene);
// Returns X and Y of canvas... top-left screen being (0,0)
var cartesian = scene.camera.pickEllipsoid(myPosition, ellipsoid);
// Takes the X and Y based on canvas and converts it to x,y,z of the map
var cartographic = ellipsoid.cartesianToCartographic(cartesian);
// Takes x,y,z of map and converts to long,lat,height