The trouble is, you cannot grow just one zucchini. Minutes after you plant a single seed, hundreds of zucchini will barge out of the ground and sprawl around the garden, menacing the other vegetables. At night, you will be able to hear the ground quake as more and more zucchinis erupt. ~Dave Barry
It's too early to start most vegetable seeds, at least for those of us in Zones 3, 4, 5 and probably 6. The rule of thumb is to start six weeks BEFORE the planting date of most vegetables. Or eight weeks ahead of time for slow growing vegetables like peppers. So, in Zone 5 we can put tomatoes out around the end of May, without protection. I could start tomato seeds the first week in April and that would be about the right time.
There are always exceptions to the rules of course. If you want to start celery, leek, or onions from seeds instead of transplants then you want to start those in February. Or try your hand at growing begonias and petunias from seed. You would then start those in February too. Be aware that you will need a VERY sunny window, sun room or greenhouse to grow these plants. They will need to be turned daily so they don't grow towards the light and "bend". I'm not trying to discourage you, just letting you know to plan for the room and the work involved.
Some vegetables can be planted directly outside. These vegetables can be planted a week to two before the last frost, as long as you can work up the soil, which shouldn't be a problem by then: beets, carrots, parsnips, peas, radishes, Swiss chard and turnips, arugula, mustard, cress and leaf lettuce. When you are sure the weather is warm enough so you won't have frost any longer you can plant these seeds: beans, cucumbers, nasturtiums, basil, dill, corn and squash.
Here are the posts from last year that have details on seed starting:
Starting Herb Seeds Indoors
Growing Lantana from Seed
Pumpkin & Sunflowers from Seed
Tomato Seed Starting Tips
Damping Off (Seedling problems)
Sowing Tiny Seeds
I'm sure I'll think of more seed sowing tips, but if you look these over and have more questions please let me know.
Watch this Important Documentary on Seed! Available for a Short time. - Dear Folks, No discussion of food can be complete without the talking about seed. [Pictured is my saved Egyptian Spinach, Garlic Chive and Roselle seed.) ...