it is very early to be growing them. If you have sown seed then you will have to wait until they are warm enough to germinate.

This is the way I start my tomato seed. I sow into a half-size seed tray with the seeds on the surface, lightly water it and then put a pinch of compost over each seed. I put a layer of clingfilm over the top to hold in the moisture and put it in the dark of the airing cupboard. After four days, start checking and as soon as you see the first white shoot breaking through, bring the tray out, remove the clingfilm and put it on a bright window sill.
I've been using this method for over twenty years and get something like 80% germination success rate. Good luck.

I'll be trying that Andrew.