Center of the screen

Good morning, every one.

How do I put the coordinate (0,0) in the center of my window.
When I draw something it apears on the bottom of the screen.


Here you go

Thanks friend, but when I did this one of my drawings disappeared

Be careful. The screen coordinates with this method are x -1 to 1, y -1 to 1. You probably need to scale you coordinates to this range. (In fact that is what the projection and view matrices are for).