A soft shell means that your turtle did not get enough calcium.
There are two possible reasons.
1. You didn't give enough calcium in the diet. You can fix this by providing calcium. You can get a cuttle bone from the pet store and scrape bits of it into the food for your pet.
2. You didn't provide enough UVB light. This is direct sunlight -- NOT light through a window. If your turtle is inside, you need a get a UVB light for the turtle tank. Here is a link about lighting.