First, make sure that your plants actually do have tomato blight. On leaves, brown to black spots form and enlarge, developing concentric rings like a target. Heavily blighted leaves dry up and die as spots grow together. Lower leaves are usually attacked first. Targetlike, sunken spots will also develop on branches and stems.
If you have not planted your tomatoes yet, then you can easily control tomato blight. The safest option is to plant your tomatoes in a different location from last season (50 feet away and in the sun.) In the old garden you could plant onions, garlic, or another resistant plant. You could also try to plant the tomatoes in the same site: First, remove all plant debris from last growing season - diseases spend the winter in dead plants. Turn over the soil and allow it to sit in the sun for a few days (repeat this process several times - it's called solarizing the soil, and it kills many of the bacteria, fungi, and insects that are present.) When you buy seeds/plants, be sure to buy varieties that are disease resistant, or specifically blight resistant (Alternaria blight is the formal name for tomato blight). You may also want to soak the seeds in a disinfecting solution before planting.
If blight appears during the growing season, control is somewhat more difficult. Cut off affected branches as soon as you spot signs of blight. If only one of your plants is affected, remove that plant from the garden.