Once you have your selection of seeds at hand, look at the length of time until maturation. You want to make sure that the time when your plants will be ready to set out into the garden coincides with warmer nights when the temperature doesn't drop below 45F or 7C. Starting your tomato seeds approximately 6 weeks before this would be ideal.
You can start seeds in everything from egg cartons to fancy germinating trays. The main thing is to keep the seeds moist, but not molding, with bright light during the day and cool temperatures at night. If the night temperature is too warm, your plants will shoot up too quickly and be thin and spindly. The How To Grow Tomatoes From Seed section will give you all the details, as well as some products you might like.
From there, just follow the links at the bottom of the page from starting your seeds, through transplanting into larger pots, setting your tomatoes out into the garden, helpful hints on staking and pruning, right through to tomato plant pests and diseases.