Depending on where you are. Timing is very important. Type of onion is very important. Fertility of soil soil is important.
Start with a well prepared soil. It needs to be loose and friable. Onions are heavy feeders especially of nitrogen. They are essentially a leaf vegetable. The onion bulb is essentially the base of the leaf.
If you you are in the north, use long day plants like Spanish types which grow large. In a border states use a day neutral plant like Candy.
In the deep south use short day plants of the Granex or Grano type. These need to be winter grown, so transplant in late fall (November- December) for best results. The other types other types for mid and northern states as soon as the ground can be worked. That means after the spring thaw and ground has dried out a bit. Depending on your area that can be late winter-early spring

Good advice from farmerdill.

TwylaG, if you want more advice let us know where you are planting and when. As said above, timing is very important.

I am planting in anywhere from zone 3 to zone 1a - depending on which zone map you look at. EVeryone else here can grow onions fine. The kinds I don't really know any names; there's the bulk bin at the co-op and there's been sets from the seed displays at canadian tire. Pretty much generic and common and what everyone else here plants. I change the spots in the garden every year in case it was just in a bad location like too near some bushes at the edge or too much sun in the middle etc.

I, too, had problems with onions. I planted a ton of them, and the biggest were only about two inches across. Bought them out of the big bins at Home Depot. It was pretty frustrating, but I eventually found out that I had bought short day onions, and I had planted them too late. This year I am planting a Spanish type onion, and hope to get them in the ground as soon as I can.

But a question of my own... farmerdill, you said that onions are heavy nitrogen feeders. Will a fertilizer be necessary, and when? Fish emulsion? Sea weed? I only use compost and composted manure usually.

Kim

In your case, add some bloodmeal, chicken manure, fish etc. something with a naturally high nitrogen content.

Okay, that makes sense. Will this adversely affect other crops surrounding the onions? And is this beneficial to shallots (green onions) as well?

Yes it is beneficial to shallots and to any leafy vegetable or grass (like corn). You can overdo for root crops and fruiting vegetables, which results in excessive plant growth and low production. Not too much danger with the low concentrations found in the named materials tho. With onion family top growth translates directly to bulb size.

So, if you're growing from seed in the south, when do you plant the seed for the short day type? Spring or fall? I'm thinking about planting seed in the ground but have no idea if it goes in my spring, summer, winter or fall garden here in Phoenix.
I've had great luck with green onions through the cool months but haven't ever tried to grow full-sized onions.

Here, They are seeded in early September, transplanted in November, harvested in April - May. They can direct seeded as late as mid October.