So, one of the problems that you may witness in your tomatoes is blossom end rot. This condition is usually caused by uneven watering or lack of calcium in your soil. You can see a brown and mushy spot where the blossom of the tomato flower would have been.
Sadly, you will most likely have to remove the affected fruit, as it is likely to rot further as the fruit continues to ripen. However, you should be able to fix the problem for future fruits/
First, water your plants on a regular schedule. Try to ensure they are receiving an even level of moisture.
Photo from http://thegoodthing.org
Secondly, there are varying suggestions on how to do it, but you need to get the plant some more calcium. One resource suggests adding eggshells to the top of your soil, seems like a simple way of doing it?
Here are some other sites you can check out: