Salut,
il faut que tu utilises un object Graphics puis les méthodes DrawEllipse ou FillEllipse.
Exemple :
public void DrawEllipseFloat(PaintEventArgs e)
{
// Create pen.
Pen blackPen = new Pen(Color.Black, 3);
// Create location and size of ellipse.
float x = 0.0F;
float y = 0.0F;
float width = 200.0F;
float height = 100.0F;
// Draw ellipse to screen.
e.Graphics.DrawEllipse(blackPen, x, y, width, height);
}