I'll try to make this as simple as I can, but it does take some time and patience or your efforts won't be rewarded.
Because of the nature of the tomato plant, the seeds won't germinate unless they've been through a fermentation process, which, normally would happen on the vine if left to their own devices.
So... pick several of your best fruits since they will carry the best genes for next year. Cut them intwo sideways... that is, the stem end and blossom end will be left, rather than length wise. Scoop out the middle since it has the most seeds. Place the pulp, seeds and all in a glass jar (so you can watch the process) and then leave it in a nice warm (but well ventilated) place for two or three days. The mess will begin to germinate and bubble up. That's a good thing! (disregard the smell or cover it with a cheesecloth to help). Then, a mold will develop on the pulpish mess. After all bubbling has finished scoop off the mold layer. It usually just lifts off in one piece. Rinse the seeds well under cool running water I use a sieve). Place on a glass dish (don't use paper or paper towels since they will stick). Let them dry, moving them about several times for a few days, place in a labeled envelope (I like to write the variety, date and planting location on the envelope) and voila, next summers goodness in an envelope... Best of luck!