Don't get discouraged. All the information you need is right here:
http://www.almanac.com/plant/lettuce
and here
http://planthardiness.ars.usda.gov/PHZMWeb/
When you're on the farmer's almanac site, you'll notice lettuce will grow in zones 4, 5, 6, 7, 8, 9. Then go to the map, and see what zone you're in. It will basically grow everywhere, and it likes partial sun coverage. This might sound funny, but if you live in a place where forests can grow, lettuce will be an excellent crop. The reason is that lettuce is a ground cover crop, and would be found under a forest canopy in nature. That's why it likes partial light, because the light that falls through the holes in the canopy, are good enough. In a open area, some screening will simulate that environment.
Just remember in the summer heat, you'll probably want to run your
nutrients at about 75-80% of maximum EC or TDS of your plants. If you have a EC pen, keep track of the EC. If you see it rising, then you need more diluted nutrients. If the EC is dropping, increase the concentration. Dropping is better than rising. The reason you dilute in the heat is because plants will start taking up extra water to cool themselves to optimise photosynthesis. The problem with the extra water usage, is that the roots block salt uptake, and the salts condense. When the nutrients become like sea water, the plants die of thirst, just like us.
Just think of the varieties of lettuce you want, and check out where they thrive. If you can create those conditions, start growing your lettuce.
Good growing.