If you bought sets they are probably a bulbing variety of onion. You would need to thin them and let them form bulbs. But they might not do so at your latitude, especially if they are a "long-day" variety.

If the onions are short-day onions (likely if they are sold in FL) and if you thin them to at least 3 inches apart, they may bulb for you. That's what they are supposed to do. You don't need to do anything to the tops. When the onions are finished growing, the tops will fall over.

Thanks guys, I will thin and wait.

If they are allium cepa, they may bulb. But mostly what are sold as green onions in supermarkets (aka allium fistulosum, Japanese bunching onions, negi or welsh onions) are bunching onions, and will not bulb or only very slightly. If they are allium fistulosum, they can be grown as cut & come again perennials.

Here is a link that might be useful: http://en.wikipedia.org/wiki/Japanese_bunching_onion