You squeeze a good number of seeds with their jelly-like coverings into a small container, put in some water, and let them ferment a few days, until the jelly-like substance has been dissolved. Then you pick out the seeds and lay them on a paper towel, and let them dry. If you can, you place them evenly on the towel so they can be cut into neat little squares with one seed on each piece. When they are dry, you can roll up that paper towel (don't cut them into squares until you are ready to plant them next year) and place it in a cool, dark, dry place, like a storage room, or a box on a shelf, in a place that doesn't freeze in the winter. Next spring you cut up the squares and plant each square in it's own pot, water and set it in the sunshine. You don't need to tear the seed off the paper towel, as it might damage the seed. But like JoyaSee said, if your tomatoes were hybrids (check the label off the pot you planted from the hardware store), they likely won't be as good. It's all about genetics. That's why heirloom varieties work better. But if you know what variety you bought from the store, you can look for seed packets in the stores. Atwoods usually has a superb collection in the spring. I often pick up seeds on sale in the fall when the stores are clearing out their garden sections. Placing them in a box in a cook closet will keep them nicely until next year.