Dying Tomato leaves
It looks like you may have a case of Early Blight, a disease caused by the fungus Alternaria solani. It is common on garden tomatoes and usually shows up around now when the plants start to set fruit.
Symptoms typically start on the lower leaves as tiny dark brown spots which enlarge. As the disease progresses, leaves turn yellow and the spots make them appear "freckled.” Eventually the leaves turn brown and drop off. Black pycnidia (fungal fruiting bodies that appear as pinhole sized black dots) form in the center of the spots as they mature.
Figure 1. Yellowing of lower leaves with brown " halo target" marking from Early Blight. [USDA]
The best thing to do is remove the infected foliage, trying not to knock spores from those leaves onto uninfected leaves. As the disease is favored by overhead irrigation, water your plants from below. You can remove 8" to 12" of the bottom foliage to help you with that. Depending on how big and leafy your plant is, you might remove some of the extraneous foliage to allow more sun and air into the center of the plant (not too much or the fruit will get sunburn!).
In the garden, the fungus can over-winter on diseased plant debris so it is important to clean up and discard infected plant material at the end of the season.