Last year I was a little too excited and planted my tomato seeds in trays in early February, but this year I am waiting until early March because the weather just wasn't conducive to putting in the greenhouse so early. The second weekend of March I will sow the seeds in trays of seed compost and vermiculite. Seed compost has very little nutrients in it, because seeds don't need much feeding at this time, but you must repot them in richer soil later or your plants will starve! Vermiculite helps to stop the soil from drying out, and helps the roots to grow strong. Both are available from all good garden centres or DIY shops, and aren't too expensive.
Once seeds have been sown, keep at a temperate of 18-21c to allow for germination. I put mine in the dark in the warm airing cupboard for the first few days, and then move to a warm window sill. Don't let the seedlings grow too tall, you can avoid this by allowing them as much light as possible so they aren't growing trying to search for light. When the seedlings are big enough to handle, thin them out leaving the strongest, and plant in bigger pots of good rich compost. Transfer to the green house when all risk of frost has gone, or put outside a few weeks later than this, in a very sunny position.
I will put mine in the ground bed of the greenhouse eventually, making sure they are tied to tall canes and that the little side shoots are pinched off. More detail to come later...with pictures to help!