Hi do i need to make a reservation we plan to visit on the 29th of June 2014. And wish to see both the wall and the water city.

Well, I think it's will be too difficult if you don't understand Chinese because the reservation should be done on its Chinese official website. You may call 010-81009999 (service line) and see if they can help you. Here is another solution. You just buy the combo ticket for the wall and the watertown together. Then you needn't make reservation in advanction.

How does one buy a combo ticket? Can it be done on site, or does it need to be done in advanced?

Patrick, you can buy the combo ticket when you arrive there.