1. If you live in an area that has long summers, you can use seedlings. Plant them in rows about 3/4 of an inch deep. When they come up, thin them so that they are about four inches apart.
2. If you're going to use seedlings, plant them four inches apart. Bury just the bottom root section - not the green stuff.
3. Planting from sets is probably the most popular way to grow onions. Plant them in rows 4-6 inches apart. Dig a small hole to fit the bottom part of the bulb. If you just press them in, they might pop out when they begin to grow. Leave the neck part of the bulb above the soil line.
4. Plant in full sun and good soil. Make sure they get regular water. When you plant them, the ground should be moist, but not super-saturated. (Here in Cincinnati, it's been raining a lot, so I'm going to have to wait for the ground to dry out a little before I plant mine.)
5. As far as varieties go, some work better for northern climates, and some work better for southern. Pick which is best for your area. Note: Yellow onions can be stored the longest.