How do I make a bow?

I can't get my bow to work. It's shooting the arrows but no matter how long I draw the bow back, all the arrows shoot with the same speed and power. How do i make it work like a normal bow? I need the power and speed of the arrows to be relative to how long I draw the bow back. I also need it to not fire if I draw it back for not long enough like with normal bows. Thanks