How can i get my butten in the middle of my screen?
Many examples here:
https://www.google.com/search?q=site%3Aunity3d.com+GUI+button+middle+of+screen
Define a button size
var buttonsize : int = Screen.width * 0.2;
Then for your GUIbutton use a rect like
Rect(Screen.width/2.0 - (buttonsize/2.0), Screen.height - (buttonsize/2.0), buttonsize, buttonsize)
.