Gert, You don't have to cook them first, besides that changes the fresh tomato flavor. I used to can, but just freeze now, or dry in dehydrator. To freeze just slice or chunk to the desired sizes you will be using in chili, stews, goulash, casseroles etc. and place flat on a large cookie sheet, cover with foil or plastic wrap and freeze. Remove when frozen to plastic zip lock bag. This is so much easier than canning that I gave most of my canning jars away. Tomatoes will last many months in the freezer. I've even frozen them up to a year and they are still good. If they do collect a little frost over time, just rinse in colander under cold water before using.