Onions are from the Allium family which includes Leeks, Shallots, Spring Onions, Chives and Garlic.

Best Time to sow seed in Southern Queensland is in late March to early June. If you live in a frosty area, such as the Tablelands or beyond the black stump, sow in early spring, after the frosts.
They like the cool weather to establish themselves, warm weather for maturing.
You can either sow seed directly into the soil or as I prefer to do, sow my seeds into seed trays and transplant them out when they are about 50 to 75 mm high. With onions and leeks you need to plant them 150mm a part (6" for us oldies) if you are going to sow seed directly into the soil you will need to thin them out once they are around 75mm high (3'') and re-plant the seedlings you have just pulled out.
When you transplant them cut half the length of the roots off and the same with the top, don't bury the bulb, just the roots and give them a good watering for the first week. Onions are easy to grow. If you drop or leave an onion seedling on top of the ground it will survive by rooting down into the soil.

Not a good idea to plant the Allium family

1. next to peas and beans - plant a row of carrots in between your onions and the peas and beans.
2. in the shade - they need at least six hours of sun.

Leeks like (Allium porrum)

1. a well-drained soil
2. lots of water in the summer
3. a good drink of liquid fertilizer once a fortnight once they have started to mature
4. compost

If you like your leeks blanched you can plant them in trenches and hill them up with compost or mulch. By hilling them up it will also help to keep the weeds down and the moisture in. Another way is to cut the bottom out of a cardboard milk carton, and place it over the leek. I don't think blanching makes any difference to taste, just looks.
A good thing about leeks is you can just pull a couple of leeks up when you need them for the pot, they don't need to be cropped and dried out like onions.
The only thing I don't like about onions is they take up to 200 days to mature and take-up lots of space in the garden, but it's great having your very own onions to eat and pickle.
The varieties I plant are Autumn Giant and Musselburg from Eden Seeds

Onions (Allium cepa)

Don't like

1. To be overfed as they tend not to bulb-up (lush tops and no bulb).
2. Lots of water once they start to mature, as they tend to rot.
3. Weeds - you will need to keep you onion bed free from weeds.
4. Keep mulch away from Onions as the bulb needs to sit on top of the soil, not in the soil. Once the bulb starts to form, with your fingers, scrape away the soil around the bulb. This will help it to develop a more uniform bulb. It will also stop the bulb from going rotten
5. Don't let the onions bolt to seed, bend the tops over, this will stop them from growing.

When to Harvest Onions, when the tops start to dry out, it's best to fold (kink) the top of the plant (the stem) over and tie it up with one of its own leaves. This will help the onion bulb to dry out quicker. Harvest them when the onion tops have dried out and the onion just pops out of the ground; when it's very easy to pull out and you don’t have to force it. Don't break off the tops, as you can use the tops to hang them up for storage.
The varieties I have planted out are Locker Early Whites and Gladalan again from Eden Seeds who also have Bunching Onions, Red, Brown and White Onion seeds for sale in their new 2000 Catalogue.

Saving Seed I prefer to buy in fresh seed as it is a hassle saving onion seed. It also takes up good growing space in the home garden.
Saving onion seed takes two years, the first year is the actual growing of the onion bulb, year two is the onion bulb flowering then setting seed for the next season.

Spring Onions (Allium cepa var. cepa) are grown for their tops. They don't bulb up and can be grown all year around.
With shallots you can grow them either in groups or plant out in rows 30mm apart. It all depends on the amount room you have in your garden. They will also multiply.

Shallots (Allium cepa var. aggregatum true shallots) grow similar to Onions, they are grown for their bulbs not their tops, they are ideal for pickling and salads.

Garlic (Allium sativum) grows from a clove only, don't plant out seeds.
If you are going to have a go at growing garlic it's best to obtain a bulb/cloves from an organic supplier. Most garlic in the commercial shops is imported, more than likely has also been treated or could be diseased.
Garlic likes the same soil conditions as onions. Store in the bottom of the fridge for a week or so to break dormancy, before planting out.
When planting out your Garlic break the bulb apart, plant the cloves with the pointy end up (make sure the top is still the top) just level with the top of the soil or just above. It is a good idea to mark the spot as most gardeners forget where they have plant out their garlic, especially after it has matured.
Garlic matures in 200 days, when the tops start to dry out you can pull them, hang in a cool dry area. If you leave the bulbs in the soil they will start to re-shoot, when this happens you will loose the bulb that it shoots from, it would be best for you to pull all the crop up and start fresh using your own clove stock. Not only is garlic great in for cooking and your health, you can make an organic garden spray to help keep the bugs away.