I sent this last week by email in answer to the same question, it's good to be able to reuse!
When to start? There are some calendars and lists online for our climate, if you search for "garden tasks" and "garden calendar" for some sites with what to do by month or season. They're fun to read. And you should make up your own list, I find it's always good to write things down.
It breaks down pretty simply into four groups: indoors earlier, indoors later, direct seed cool, direct seed warm. For southern Ontario (Zone 5/Zone 4 US), it's more or less:
*START INDOORS (Jan-Apr) or DIRECT SEED (Apr-Jul/Aug). Some things you start once for the season, like tomatoes, others, you may start a first planting indoors, and then direct seed successive plantings (like broccoli, cauliflower, summer squash).
The crops I start indoors I usually keep starting in trays, even when I could direct seed later in the season. For example, I'll plant broccoli and cauliflower maybe three times, and do them all as transplants because it's easier to germinate and care for the seedlings in trays than in the field. That's a matter of choice, mainly to do with the amount of time you have for watering and weeding–you can start 100' of broccoli in just one tray, and then transplant in 30-60 minutes, including watering in.
*Start INDOORS EARLIER (Jan-Feb) or LATER (Mar-Apr). The usual veggie/herb crops roughly break down into two groups, ones that need 10-12 weeks to start and ones that need about 6-8 weeks, _roughly_ counting back from the last frost (third week of May).
The 10-12 week ones are mainly herbs (rosemary, sage, etc, they grow slowly) and leek/onions (which can also be transplanted 2-3 weeks earlier than a lot of other stuff).
The 6-8 week ones are mostly planted out around last frost (second half of May), so you start 'em around mid-March. The main exception here is cukes, all squash, melons (these are the cucurbits), they grow really quickly in pots and are best put out when things have warmed up, like, end of May, so you probably want to start most of those only around the first half of April (and in bigger pots, if you can, like, 3").
*DIRECT SEED COOL (Apr) or WARM (May). Again, a break between seed that germinates in cool soil (40-50F) vs warmer soil (50F+). The cool stuff are peas (as soon as you can get out there), and roughly, carrots, parsnips, onion family (like green onion), beets/chard, spinach, lettuce, radish and brassica greens (arugula, mustard, etc). The rest do better as the soil warms up.
That's about it for simplification! Maybe make lists with those headings.
Timing is of course a big deal, a lot of gambling, especially with our crazy weather of the last few years. The cool thing with tiny farming is you can afford to hedge your bets and take pretty big risks.
I used to try and plant everything as early as possible, but you see after a while that that's often as not NOT the best thing. Especially with transplants, if they hit a bad couple of days or week (cold, mainly), which often happens, at the very least, their growth is slowed down, and they may suffer some damage which sets 'em back, PLUS, you may have to do extra work covering them up (floating row cover).
Plant out a week or two later, and you may actually get the same or even an earlier harvest than for an early-as-possible planting! I've observed this, and read about this here and there as well, although many growers still push for early as possible.
I used to think I was doing well because I hadn't really lost anything to early planting, as in, killed dead by cold, but if the growth setback was a week or two, then I did actually lose to weather by planting too early, AND I did extra fieldwork for that period.
OTOH, for the times when you win, nothing's cooler than a super-early harvest, being first on the block by a week or two with whatever crop!
So now, I try staggering things, and holding some back-up seedlings, and don't get too caught up on precise starting and transplanting dates, do some risk planting as early as seems likely, and plant some later. You can afford to do things like that when you're planting small and often.
You can find recommended weeks-to-transplant timing for all the usual garden veggies in many books and places online (here's one I just found, but check a few, like most garden advice, sources can vary quite a bit, go for a popular average to start, then see for yourself!).
Here again, you can experiment quite a lot. Try cutting back a week or two on the minimum time–the less time indoors, the less resources you use (and the longer you hold transplants, the longer they take to readjust to the wide open spaces in the field is what I've found)! Also try using deeper pots, root depth seems to be the main boost you can give to transplant seedlings (of course, if you're doing hundreds or more transplants, this one can be difficult). But everything's worth a try, even if it's only with a few plants just to see what happens!
That's my experience so far. Hope it helps!