Follow these instructions for saving your own tomato seeds for next spring
Wash your heirloom tomatoes, then cut it in half across the middle (not the stem end). Gently squeeze tomato seeds and juice into a labeled glass or plastic container.
Fill containers about half full, then set them out of direct sun in an area where you won't be bothered by the ripening odor or fruit flies.
Full story: Why save tomato seeds from year to year?