Buy onion plants, as shown above, available at some local garden shops and in seed catalogs.
There are three main types of onions. First is the short day or Southern varieties. Avoid these. They will most likely not form large bulbs and you may be disappointed. Many of the onion sets (those small bulbs sold in mesh bags at your big box stores) are not labeled so you only know if you are getting red, white or yellow onions. Avoid these as well.The next type is the long day or Northern varieties. These include Walla Walla and Sweet Spanish Hybrid. These will grow OK. If you want truly the best, grow mid day or day neutral varieties. Larry has had great success with the yellow mid day, Candy Hybrid. It grows to softball size and stores well. If you want a mid day red, try Sweet Candy Apple Red and for a great white sweet onion, try Super Star White.
These mid day types should be ordered as onion plants in bunches through a seed catalog but first try your local garden center. Be sure your garden is ready to plant these guys. They like to planted no later than the end of April (in the Colorado front range). The frosty cold nights ahead will not hurt them.
The sweet mid day onions mentioned above will not store much longer than two to three months. For a good long storage onion also try Copra and Red Zepplin.