If you've always wanted to start your own tomato seedlings rather than purchase plants at a nursery, here are a few pointers. We live in gardening zone 7 and have a greenhouse so are able to get a jump on seed starting for our area. Let me start by saying we grow on a largeish scale, but you should be able to downsize to fit your needs. This process works for any kind of tomatoes, but we grow only heirlooms for a few reasons; they taste so much better than hybrids, my customers at the farmer's market want them, and you can save their seeds to plant next year. Here are some of last year's harvest.
We don't have large dinner parties between New Years and Easter because our dining room becomes seed-land. We lay down shower curtain liners and set up a utility table on them. We hang shop lights both above and below the table's surface. We don't use expensive grow bulbs, just regular shop lights. They're on a 14 hour timer. We have no (zero) sun in our house thanks to a wrap around porch. If you have a sunny windowsill, that will work just fine!
We use seed starting flats and a seed starting potting mix. You can find both in lots of places including WalMart and home improvement stores. Following the instructions on your seed packet regarding the time to start your seeds and the depth to plant them, plant 2-3 seeds in each of your tray's compartment. We spritz them daily with water and keep the lids on the trays until the plants get to be about an inch tall.
See how all three seeds came up in this compartment? I take my tiny sewing scissors and cut two of the three, you only want one plant per compartment. If you try to pull the other two out, you run the risk of damaging the roots of the plant you want to keep.
When your plants have three sets of leaves, it's time to transplant them. It took five weeks from the day I planted the seeds for the plants to get this tall.
If you've planned your planting dates such that your last frost date has passed, you can put these outside either in a planter or into the ground. We're still two months from our last frost date so these are going into one cup sized peat pots. I write right on the pots with a sharpie. These are brandywine tomatoes (BW). I'm also experimenting this year with wooden swizzle stick plant markers. I'm writing on those and sealing my writing with clear nail polish. We'll see how it goes.
Before you try to remove them from their compartment cells, water thoroughly. This will hold the roots in the potting medium and make the plant easy to remove. I use a fork to remove the plant from the cell and place it into the peat pot. Never touch the stem of a plant, just the dirt and leaves if you must. For tomatoes, it's important to plant them as deep as possible. The stems will grow more roots and improve the stability of your plants. I fill the pot with a 50/50 mixture of seed starting mix and potting soil with a few Soil Moist crystals thrown in to help maintain moisture. They're hard to find and expensive but we use them. I've found them both online and in a very upscale garden center.
Here a photo of our back deck from last summer, just to show you it's possible to have tomatoes in pots! We grow about 50% in pots and 50% in raised beds. If you have any questions about our process, I'll do my best to answer.