go to the property window of your main form and set WindowState to 'Maximized' to make your program start in full screen. In order to make the buttons stretch, you have to play around with the anchor property of your buttons. If you want your button to strech to the right when the windows is resized then you set it to right. Same for all four directions.
or if you want to code it in then you use button1.anchor = 'anchor_style'
hope this helps