How to control Late Blight on my tomato tree?

Over the last month I have noticed that the leaves on my tomato tree are turning black and I now believe it has Late Blight. The tree at this time has about 20 tomatoes still in the growing process and although they look great...the leaves are sick. I bought a fungicide for the tree but am afraid to use it wrong. How much do you spray and where on the tree? How many times a week and for how long? If there is anyone out there that can tell me how to take care of the plant and get it back to being healthy, I would really appreciate it. I got the plant from Costco if that matters at all. Thanks