Help my poor tomato plants!
There are several fungal leaf spot diseases that commonly infect tomatoes. This could be early blight (extremely common) and/or septoria leaf spot.
If the yellow leaves are on the bottom on the plant, simply pull them off and dispose of them. These fungal disease spores splash up from the soil, so a good mulch of, for instance, newspapers covered with leaf mulch or straw, keeps the spores from splashing on the lower leaves.
Other strategies may be necessary. See our website info in the Grow It Eat It section devoted to vegetable growing for more detail: http://extension.umd.edu/growit/vegetables/diseases-vegetables
Also, we have a good fact sheet on tomatoes with a handy diagnostic chart of symptoms: http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG84_Pruning%20ornam...