I got my seed orders Thursday and Friday, and my plan is to start onions in the next few days. This web site is a great resource for knowing when to start seeds. I don't follow it exactly, but it's a pretty good approximation of what I do, and you won't go far wrong if you do follow it.
Tell the calculator that you're starting your spring garden, then choose whatever planning date you want (like today's date), and set your last spring frost date. I set my last frost date for May 18. It tells me I don't need to start anything until Feb 23, which is 12 weeks before my last frost date. It suggests that I should then start some of my cabbage family, head lettuce, onions, and parsley.
My own bias is that onions should be started a little earlier than that, and I start the cabbage family a little later. Here in the north we can only grow what are called "long-day onions" successfully. This means that these varieties are triggered to start making a bulb when the days get long at the end of June. So it's important to have as much growth as possible before the days get their longest. It's a wonder we ever got any onions at all when we planted on memorial day weekend. (There are also day-neutral varieties, which may or may not perform as well as long-day onions.) Local stores will have selected the right day length for your area, so no need to worry about that when choosing seeds - but pay attention to the descriptions if you're ordering froma catalog.
Onions are actually easy to grow from seed if you're starting other plants anyway. They spend a long time looking like grass, so don't be discouraged. When they first germinate the seed itself will stay attached to the end of the onion. The first time I saw that I thought perhaps I hadn't buried the seeds deep enough, but that's just what onions do.