Tomatoes are typically sown 6 to 8 weeks before the last anticipated frost. Some say to sow them later, more like 5 weeks before the last frost, but either will work. Keep in mind that the earlier you start, the larger the plants will be when you plant them out.
To find your average last frost date, go to http://cdo.ncdc.noaa.gov/cgi-bin/climatenormals/climatenormals.pl?directive=prod_select2&prodtype=CLIM2001&subrnum%20to%20Freeze/Frost%20Data%20from%20the%20U.S.%20Climate%20Normals
I live in Middle Tennessee and start my seeds in mid- to late-February.