How do i set this up? i don't know how to do these.

You basically FOIL the letters like in math. First, outside, inside, last. For example if I have the dihybrid cross BbGGXBbgg then I would but the first letters of the first set (BbGG) together (BG), the outside letters (bG) the inside letters (BG) and the last letters (bG). Then you take the next set (Bbgg) and do the same thing so first letters (Bg) outside letters (bg) inside letters (Bg) and the last letters (bg). Then after that you take each of the letters and put them down like a normal 4 square punnet square.