Carol, this question comes up every day, or twice a day on every forum. Dr Carol has answered it so many times that she is probably about to give up talking about it.
Because it is a calcium shortage people thing that egg shells or such will help--doesn't work that way. Very little you can do now. Try to reduce any kind of stress on the plants and water care fully. It will usually stop as the plants get older. It's been a bad year for it because of the weather.

Do a GardenWeb search on BER as well as the Tomato forum FAQs.

I guess it is a lack of calcium, but now that my plants are growing and I have fruit, what can I do to fix the problem.

No it isn't a lack of calcium per se, not soil calcium at any rate. I think most all of the many discussions on this forum about BER as well as the FAQ here on BER make that point well. It is inconsistent distribution of the calcium to the fruit during development because of inconsistent soil moisture levels.

The "fix", if you want to call it that, is time, patience, and consistent soil moisture insofar as possible. Avoiding things like using containers that are too small, avoiding varieties that are very prone to BER naturally, and not over-watering plants can also help.

BER goes away all on its own as the plant matures and adapts.

Dave

Here is a link that might be useful: BER FAQ and discussions