Starting tomatoes in the house is a good idea. Very often we start them in a window which doesn't really give them the light they need and they grow spindly stems and roots. They really benefit from more direct light from either grow lights or under florescent bulbs (one warm, one cool). But anyway, once the seed has germinated and you see the second set of leaves developing, you need to transplant the whole thing, dirt and all, carefully, into a larger pot of soil (think from 4 inch to 6 inch in size). Tomatoes have large root systems and even as babies, need to be transplanted several times before being ready to plant outdoors. In the greenhouse, that's what they have done plus the light and atmosphere is totally perfect to help them grow strong roots and stems. Another thing you can do as well as transplanting in larger containers as they grow, is to lightly brush your hand over the plant occasionally much as would happen outside in the air. This helps to develop stronger stems and roots. Once you are ready to put them in the ground on or about May 15, make sure you have dug the planting hole deeply and added compost then bury the plant in the soil right up to the leafy part. The stems have growth nodes on them and they will sprout more roots under ground and the whole plant will become stronger.
Broccoli is a cool weather vegetable and it is often more productive to purchase plants from the nursery than to try starting them in the spring yourself. Most of those at the nursery or garden center will have been hardened off and are garden ready and should begin to be available now or in early March. If you have started them in the house, you definitely need to give them time to adjust to the outdoors by setting them outside everyday for awhile - now is a good time to begin doing that - and do it for about a week or 10 days - then plant them in the garden in the evening or early morning. In late summer, you might just want to plant the seed in the garden itself in late August, early September.
Most cool weather vegetables, when the weather becomes too warm, will almost always bolt - that is to say they will bloom and go to seed very quickly and the heads of broccoli will be small and sparodic on the stems before going to seed. So, planting them early - (begin now), should make all the difference in the yield.