Tomato plants are easily grown from seed, especially with the assistance of a heated propagator to germinate them. Google will find you instructions suitable for the area you live in. In climates with frosty winters, we sow the seeds indoors about 6 weeks before it is safe to put the plants out, which is once all risk of frost is well past. Though if you have a greenhouse, then you have a wider range of options.

But I would not in general suggest taking seeds out of a tomato you bought in the shop, because many of them are hybrids and won't breed true, and might not have ripened sufficiently on the plant to be good seed. You'd do better to go to a seed seller and get tomato seed of varieties suitable for home cultivation in the area you live in.

But if you want to try taking seeds out of a tomato, etc, then this seed seller is pleased to tell you how to do it. Great Vegetable Seeds from The Real Seed Catalogue