How can i make change the First Person position by clicking a Buttom?

Take a look at this question. It might be related to what you want. Which object you want to change the position of depends on the way you have set up your object hierarchy.

To get whether a button (spacebar) is pressed, you can use Input.GetKeyDown(KeyCode.Space)

Check out http://unity3d.com/support/documentation/Components/GUI%20Scripting%20Guide.html for a guide on UnityGUI if you haven't seen it yet. I would do something like:

function OnGUI () 
{
     if (GUI.Button (Rect (10,10,150,100), "MOVE")) 
     {
          Move();
     }
}

This will draw the button and call the function Move() when it is clicked. Now in your Move function you place the code to move the object.