I have a 2013 Macbook Pro with Retina display and a 2008 iMac 8.1, and I want a dual screen monitor with my Macbook Pro as my primary. What cords do I need in order for this to happen?

I have a 2013 Macbook Pro with Retina display and a 2008 iMac 8.1, and I want a dual screen monitor with my Macbook Pro as my primary. What cords do I need in order for this to happen? I do a lot of photography and graphics work and could really use the extra screen space, and it would quadruple my output time. I just don't know which cords to get in order for this to be possible.

Thanks for any help you're able to give!