// Method 1: Get the start point of the polyline by converting the polyline
// into a collection of points and getting the first point
// sketchGeometry is a Polyline
// get the sketch as a point collection
var pointCol = ((Multipart)sketchGeometry).Points;
// Get the start point of the line
var firstPoint = pointCol[0];
// Method 2: Convert polyline into a collection of line segments
// and get the "StartPoint" of the first line segment.
var polylineGeom = sketchGeometry as ArcGIS.Core.Geometry.Polyline;
var polyLineParts = polylineGeom.Parts;
ReadOnlySegmentCollection polylineSegments = polyLineParts.First();
//get the first segment as a LineSegment
var firsLineSegment = polylineSegments.First() as LineSegment;
//Now get the start Point
var startPoint = firsLineSegment.StartPoint;