It's time to get those onion sets or transplants in the ground. Above you can see my yellow and purple onion sets, ready to be covered over with an inch of soil.
When planting onion sets you will come across mildewed bulbs (above left). These should be discarded. Choose bulbs that are clean and solid feeling (above right). A small (~1/4") sprout coming from a bulb is acceptable, but bulbs with long sprouts already on them (above middle) aren't a good choice.
If you follow the label instructions on the sets, onions are pretty maintenance free. However, if you desire more information here's a great resource for onion growers: http://aggie-horticulture.tamu.edu/archives/parsons/publications/onions/oniongro.html