You can ask the camera where any point in world space will be in screen space.

Try this to get a feeling (untested code, might contain errors):

public class Example : MonoBehavior
{
    // Bind target to the object it should follow.
    public Transform target;

    // We use update to obtain the position in this
    // example so we need to keep it until OnGUI.
    Vector3 point;

    void Update()
    {
        // Find screen position for target
        point = Camera.main.WorldToScreenPoint(target.position);
    }

    void OnGUI()
    {
        // In case point.y is inverted, just subtract it from the screen height.
        Rect rect = new Rect(point.x, point.y, 100, 50); 

        if (GUI.Button(rect, "Button")
        { 
            // Button clicked.
        }
    }
}

http://answers.unity3d.com/questions/5045/using-a-gui-button-to-stop-a-gameobject-movement

hi, i didnt understand where do i have to subtract the the point.y im nbot sure if im using scree height i have this:

 void Update()
{
    // Find screen position for target
    point = Camera.main.WorldToScreenPoint(target.position);
}


 void OnGUI()
 {
 float yPos = 5.0f;
 float xPos = 5.0f;
 float width = ( Screen.width >= 960 || Screen.height >= 960 ) ? 100 : 100;
 float height = ( Screen.width >= 960 || Screen.height >= 960 ) ? 100 : 100;
 float heightPlus = height + 0.0f;



 // In case point.y is inverted, just subtract it from the screen height.
    Rect rect = new Rect(point.x, point.y, 100, 100); 

****** I've tried change the screen height that appears in the code but, that is for some other static GUI buttons.

And one more question, how do put a position for example if want that the GUI button follow the object but in a specific position (a square object and the GUI button follow it but always in the corner)

appreciate the help!