Follow these instructions for saving your own tomato seeds for next spring
Sprinkle seeds onto a plate and allow them to dry for one to three days, depending on the weather. Keep them out of direct sun. To make sure they dry thoroughly and don't stick together, stir twice a day.
Store dried seeds in a cool, dry, dark place in individually labeled airtight containers such as glass canning or baby food jars until planting time next spring.