First determine the length of your growing season - the number of days between your last frost in spring and your first frost in the fall. Read the seed packages or plant tags; somewhere on there, it will state "days to maturity" (or close to that). The number of days until you can expect tomatoes to pick starts counting when you put your little plant in the ground.
So if the package says "100 days" and you plant on June 1, you can expect to pick your first tomato in the first week or so of September. I live in zone 6 and my growing season is 135 days long so I look for tomatoes that have a growing season of 65 to 70 days at the most. I want to pick more than one tomato before the frost kills them.
If you go to the left hand bar on this page and click on "Goodies", you can find your hardiness zone and if you follow the link to the "gardening calendar", it will tell you the length of your growing season. The information you find there is just a guide as weather is unpredictable and you may have other factors influencing your particular climate.
Daisy