Although some seeds grow better started directly in the garden, tomatoes do best when started indoors where you can give them additional TLC to get them off to a good, strong start. You'll need to provide the basics to get your seedlings started-namely, light, water, soil, and warmth. Light can come from a window that admits bright light but not direct sun. Even though light coming in a window may seem bright to us, it may not be intense enough to sustain seedling growth. If your seedlings are spindly and elongated, provide artificial light. Set a timer so the light is on for 16 hours each day. Water is essential for seedling growth, but overwatering can mean the end of your seedlings. Reduce this risk by irrigating the seedlings only when they need it. Touch the soil, but also be sure to poke down a half inch or so; the soil on the surface can be dry and the soil underneath moist.
Here's another tip: Fill your tray with seed starting mix and feel the weight of the tray before you water. Moisten the seed starting mix and check the weight again. It will feel heavier. As your plants are growing, check the weight of the tray; when it starts to feel light, it's time to water. Soil is a bit of a misnomer when we talk about starting seeds. Although you can plant seeds in soil, it's best to use a sterile seed starting mix. That eliminates the risk of unwanted soil organisms growing in the environment you've created for your seedlings. You can find bags of this mix at your local nursery. Seedlings also need a warm growing environment for getting started. A location that's 65-80?F day and night will be just right for your tomatoes, as well as any warm-season crops you want to grow (cool-season crops like it cooler to start). Once the seeds sprout, they grow better in a slightly cooler environment, so turn the thermostat down 5 to 10 degrees.