For onions the first thing you have to know is whether to plant short day, long day, or intermediate day types. The process is different depending on what type you need to grow. In the link below there is a handy graphic that helps explain what grows best where.

Garlic is planted in the Fall and left to overwinter. When to plant in Fall depends on your location. What variety to plant also depends on your location.

Rodney

Here is a link that might be useful: Onion Types