If you have any interest in growing tomatoes other than the half-dozen available at garden centers, starting seeds is a good way to get variety. Seeds are available for Heirlooms, old fashioned and new hybrids. There are so many seeds in an envelope, sharing with another gardening friend is a way to double the types you grow.
In response to a question about when to start tomato seeds, Renee Shepherd of Renee's Seeds said, "Generally speaking, the goal is to start your tomato seeds six or eight weeks before the last frost date or another guideline would be that they are ready to plant out when temperatures are regularly in the 50s both day and night. I prefer this second criteria, as it gives leeway for local conditions."
For me, pictures speak louder than words. Follow this link to Renee's Garden Seeds instructions on growing tomatoes from seeds - with photos of every step for us visual learners. The photos go from first seed planting, to dividing seedlings, planting in the ground and pruning. Check it out.