Its probably Blight(a picture would help, if you can post one), you can use a preventative spray called Bordeaux mixture or Dithane, available at your local garden centre. But this will not reverse the effects that are already showing.
I have a few tom's on a couple of plants that developed some blight, i removed any infected fruits and leaves. Then sprayed. The infected plants have still produced a good crop of healthy tomatoes.

Follow the instructions/advise carefully when spraying.

Don't compost any leaves or stems with blight, as the spores will stay in your bin. Burn them or bin them!