Growing tomatoes from seed takes time and care, but ultimately it is not hard and the results are well worth it.
You will need either a grow light setup (cheap shop lights) or a south-facing window that gets plenty of sun (if you live in the southern hemisphere, you will need a north-facing window).
Use this instructable as a guide to growing your own tomatoes from seed.
UPDATE: Check out my new Web site on growing tomatoes. Grow Your Tomatoes
Step 1: Get the seeds
Research the seed company you are buying from to make sure they are reputable and that you will get good seeds from them.
I'm an heirloom grower, but you can choose to grow either heirlooms, hybrids, or some of each.
Step 2: When to plant the seeds
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.