Growing tomatoes from seed is difficult. Its better to start them indoors or buy small plants.Even if you start them from seed indoors, it can be difficult to get them going.

The area of the country and general weather patterns effect the timing. I would say that you need to be prepared with some kind of system where you can shield them should the temperature drop. Like those wire things they sell to put around the plant. You can them put a bag over them if a frost is coming. Unless of course you live in an area where it doesn't freeze.

You also have to think about other things that love to eat tomatoes. We had a crazy dog once that ate tomatoes. My mom got so mad. She would scream, " that stupid dog. He always eats the best ones. Just when they are ripe". You can put blood meal out. This will sometimes keep deer and rabbit away. My mom liked to use lead, but she was a good shot. We were also out in the middle of nowhere. You can't squeeze a few round off in most cities. In Newark, New Jersey or Flint, Michigan, nobody would notice but still.

Its interesting to note that Black Walnuts are bad for tomatoes. Black walnuts produce a chemical called "juglone" through their roots which can be allelopathic to other plants. Its interesting how some plants developed their own system for shutting out the competition. The upside of Black Walnuts is that they fetch a good price if you can get one that is plankable- even better if they are veneer quality. At any rate, if your tomatoes don't grow, and you have been planting them near a Black Walnut, now you know why.

         Tomato plants are usually best started indoors about two months prior to planting them outside. It is best to use biodegradable starter pots so that the plant goes through the least amount of trauma as possible when transplanted. You will want to transplant them when the tempature is about 60 degrees and you are sure that the temperature won't dip too low afterwards. There are charts that you can find online or in seed catalogs that will tell you the best time to plant in your area. When the tomatoes grow big enough, you want to be sure and put cages around each so they have something to keep them in place. The cages are also helpful in keeping animals from eating the plants.

         Tomatoes can only be planted when the temperature is about 65 degrees F. It is important to consider where you live when planting these tomatoes; in the Northeast there is a smaller window to plant tomatoes outdoors than in warmer weather climates. If you plant tomatoes too early in the season in New England, for example, the plant could die from the cold weather. Before planting, it is recommended that you check local guidelines before planting.

         We grew tomatoes in the summer when I was little ( we live in S. Cali)... they need to be planted when the temps are about 65 F, so it depends on where you live and what kind of weather you have at what time of year. I live in Phx in the heat so I'd probably be able to plant pretty early, but make sure you won't have overnight frosts that will zap your poor little seedlings.
If you want to save the hassle, you can buy baby tomato plants at most nurseries, some even carry heirloom (like brandywine) starters...

The website below tells you some good varieties depending on the area you are in.
Check it out at http://www.helpfulgardener.com/vegetable/2003/tomatoes.html