To save on the purchase of plants, you can harvest seeds from your vegetables and flowers and plant them the following spring. Once the seed capsule turns brown, it is ripe. Cut it over a bowl or an open paper bag to avoid losing seeds by accident. Afterwards open the capsule to remove the seeds then store them in a paper envelope for the winter, preferably in a cool location.
If the seeds are in a moist fruit (tomato, pepper, squash, etc.) rather than in a dry capsule, remove the seeds from the fruit, rinse them in clean water to remove any flesh and let them dry on a paper towel for a few days before storing.
Note that harvested seeds will not necessarily give plants identical to their parent if the latter was of hybrid origin or if there were several different varieties growing nearby, because that will shuffle the genes a bit. But if the parent plant was of good quality, its offspring will almost always be great plants too.