The University of Florida has a good publication on this. I've included a link to it below. Also, your cooperative extension agent should be able to tell you the best way to deal with it in your area. Just be sure it's early blight and not something else. Treating for the wrong thing wastes your time and money and allows whatever the real problem is to get a head start on destroying your crop.

Good luck.

Sandy

Here is a link that might be useful: Early Blight on Tomatoes and Potatoes

If it is early blight, get some ortho spray and spray your plants down. It will not cure it, but will keep it from spreading. I usually spray my plants after every rain and keep lots of mulch around them, but the ortho makes a good back up. Have not lost a plant to blight in 3 years.

As vtgreenthumb mentioned, you can start applying Daconil which is the same thing as Ortho garden disease control (29.6% Chlorothalonil) at a rate of 1 TBSP per gallon of water. once every 7-10 days repeating after a good rain. It has worked very well in controlling early blight here in South Texas