how do i draw a line when i click a button

You need to handle the event Paint or override the method OnPaint of some control (could be a form) where you paint. You handler of OnPaint will be called with event arguments parameter which provides the instance of Graphics you use for drawing.

That will create some static rendering. How to change it on some event like button click?

Make your rendering depending on some data, which can be a member(s) of you form, for example. Your click event can modify the data, so next time your control is rendered it will show some different picture. For example, no line before click, a line after click. But changing data itself does not mean re-drawing, right? How to trigger the update. You need to case the Windows message WM_PAINT to be dispatched to a control. Do trigger it, you call Control.Invalidate right after you changed the data. For performance gain, you can call Invalidate with a parameter (Region or Rectangle) to invalidate only a part of the scene. The same technique is used for animation of the rendered picture. Double buffering of the control is recommended to avoid flicker.

—SA

In your application, handel the OnButtonClick Event in that using Graphic object draw the space in required position on from.