Onions are hilariously easy to grow. Look around at Farm King or Menard's for sets (immature onion bulbs). It may be getting late in the season to find them, but you can also sometimes find seed onion plants at garden centers, but I don't have experience with them.
I usually plant onions around St. Patrick's day - about the same time I plant peas. They like deep, well-draining loose soil with good N and P. Mulch them to prevent soil compaction and make sure they get plenty of water and that's about all there is to it.
Here's a shot of my crop this season, between the lettuce and garlic.
I'm sorta a bulb nerd here. The health benefits of garlic and onions are too long to post(use the internet) and the historical and biblical importance even longer ( the Dutch even used tulip bulbs as currency) but I will tell you adding bulbs to your grow will make your grow well rounded. Bunching onions and chives make great flower bed borders always returning depending on your grow zone and type of bulb. The Egyptian Walking Onion is the Swiss army knife of onions. The white bulb (best cooked) has a stronger flavor then the standard white onion as does the green stacks have more flavor then regular green onions. The stem holding up the babies is full of Onion water (to die for if your in a desert). Add on the fact it's gona grow and populate just short of a nuclear destruction makes it the perfect starter Onion. I'd go further to talk about how you can time your grow planting to tulips but mine lied to me this year so me and it got hammered by hail last week (posted in any ks growers). Still a little pissed off with my tulip