A lot of what you need to know about the glycemic index is here. Like what IS the glycemic index, anyway? (Short answer: a system of ranking foods according to how much they raise blood sugar.) Why is the glycemic index controversial? Can eating low glycemic-index foods really help prevent diabetes and colon cancer and heart disease? And control my weight?
But not quite everything you need to know about the glycemic index is in that one blog post. Among other things, you need an actual glycemic index that lists foods according to how much they raise your blood sugar. I couldn’t find a really complete glycemic index, and maybe there is no such thing. But Diabetes.net has a good one here. (Scroll down past the book-selling stuff.)
And Wikipedia has a brief table summarizing categories of high. medium. and low glycemic-index foods. Keep in mind, though, that you can’t always easily estimate the glycemic index of a food. Those bagels up there are actually high glycemic-index items.
As with any topic related to diets, almost any search engine will garner you a ton of stuff. There’s an enormous amount of detail about the glycemic index out there, much of it useful. But there is also an enormous amount of salesmanship, so beware.