Tomatoes are started indoors 6 – 8 weeks before last frost. Actually for larger plants for the garden, you can start 10 weeks before you set them out and pot them up.
We use milk cartons and set the plant at the bottom. If you fill the rest of the carton gradually with soil you end up with a very sturdy plant since the buried stalk will develop roots.
When planting out we find they get a better start if protected with a row cover. Tomatoes need that extra warmth.
Pkt=25-40 by type G=7-14 days