As I can make a timer?

I need to execute a function after some time. How can I?

Have a review and search of the forums on the script statement:

yield WaitForSeconds(number);

The documentation for this can be seen here:

http://unity3d.com/support/documentation/ScriptReference/WaitForSeconds.html

See also this other Unity Question/Answer:

http://answers.unity3d.com/questions/262943/as-i-can-make-a-timer.html