how to minimize unused form(no event) with the help of timer

Add a Timer to your form.
Set the timer Interval to 1000 (or one second in milliseconds) and add a Tick event handler
Add a class level private integer and set it to 60 - call it inactiveTime
In the Tick handler, check the value of inactiveTime. If it is zero, do nothing.
Otherwise, decrement it by one. If it is now zero, set the form WindowState to FormWindowState.Minimized
Whenever the user does anything you count as activity, set inactiveTime back to 60.