We sell Onion Starts each spring. These are little bareroot plants that were started from seed and harvested after growing for 6-8 weeks. They come to us dormant with dried roots. However, once planted, they plump up and turn bright green.
Choose an area that gets full sun all day and prep your site by loosening the soil. Onions prefer to grow in loose, fertile soil that drains well. If it is compact add some compost and fork it in.
Onions do well in a raised bed of sorts (or a raised row). A bed or row 4 inches high by 20 inches wide is good. Plant the onions either 2 or 4 inches apart. When planting, fertilize onions with a 10-20-10 fertilizer spreading at half a cup of fertilizer per 10 linear feet. Do not put the fertilizer directly on top of the onions; instead work it in the soil about 4 inches away from the onion. If you planted your onions 2 inches apart, thin them to 4 inches apart after 45 days and eat them as spring onions. This allows the others to mature to a larger size.
Water thoroughly after planting, and as needed thereafter. Onions are shallow-rooted so they will dry out quickly. You never want the soil in your onion patch to be dry and cracked. Every 2-3 weeks fertilize the plants. Again, half a cup per 10 feet of row. No need to work it into the soil, but do be sure to water well after each application. Stop fertilizing when the onions start to bulb. You'll know the bulb process is beginning when the soil starts to heave as the onions push it away from their base.
It is important to research whether the onions you're growing are best eaten fresh or if they are good for both fresh eating and storing. Not all onions are good keepers. And nothing smells worse than a bunch of rotten onions.