Look at the MouseEventArgs class, and you will find it has a Location property - which tells you where the mouse was when it was clicked.
If you create a class level List or Point structs, then you cna just add the new location to that list for later processing:
private List<Point> points = new List<Point>();
private void Form1_MouseClick(object sender, MouseEventArgs e)
{
points.Add(e.Location);
}