I think you mean using the eggshells when you transplant the tomatoes.
Starting in late winter, every time you use an egg in cooking, rinse out the shell and let it dry. Put the dried shells into a paper bag or a bucket. You will need to store them in a rodent-proof container, because mice like to eat them. When it's time to plant the tomatoes outside, crush about a dozen shells and put them in the bottom of each hole.
Tomatoes need a lot of calcium - it prevents blossom end rot on the fruit.
I didn't save my eggshells this year. I just threw a handful of oyster shell into each hole and it seemed to work just fine.