Figure out when you want to plant them out (based on your av'g last frost and so forth and so on), then count back the appropriate number of weeks for each kind of seed. For instance tomatoes you'd usually start 5-6 wks before setting out.
For most things it's better to err on the side of starting too late than too early, unless you have somewhere good to store too-big-too-early plants that is well protected from excessive cold and gets HUGE amounts of light and good ventilation. IME most plants do better if put in the ground a little small or a little late than they do when held under poor growing conditions whilst overgrown because they got big before the weather allowed planting out.
I'm starting spinach and some speculative early lettuce this week, and also just a few tomatoes for my wall-o-waters. (I can put tomatoes out in wall-o-waters in mid April if I am careful but not put them in "normally" til the very beginning of June, we are in a frost pocket and our last frost is usually in the first few days of June)
Pat
