How can i switch to the next level after completing the task in level one?

http://docs.unity3d.com/Documentation/ScriptReference/Application.LoadLevel.html

Add simple boolean var, for example "taskcompleted" to your script and change them to true after some task, next add something like ths to update(); :

 if(taskcompleted == true)
 {
 Application.LoadLevel ("Level2");
 }

In this case Level2 is the name of your next scene to load. That's all.

The way I did it was I made a cube and set it as trigger, then add this script

 //Script for next level
  
 var levelToLoad : String;
  
 function OnTriggerEnter(hit : Collider)
 {
     Application.LoadLevel(2);
 }//END FUNCTION ONTRIGGERENTER

now just un-tick the mesh render in the inspector to make the cube transparent. When your player runs into the trigger the next level will load.