Tomatoes are one of the easiest crops to save seed from. You can save seed from any tomato so long as it is open-pollinated and not a hybrid.
To get the seed, simply squeeze it from the ripe fruit (eat the rest), add a little water and let it ferment in a warm place for a few days. Then scrape the scum from the top and rinse the seeds several times to remove bits of flesh. Strain the cleaned seed and dry it, first on paper towels and then in a warm dry place.
Generally tomatoes are self pollinated and you don't need to worry about cross pollination or isolation, you just save seed from the ripe fruit when you eat them. A few varieties may cross pollinate to some extent (to get technical you can usually tell these because they have long stigmas on the flower that extend out beyond the petals).