Apple Users

The Disk Utility that comes free with MacOS will do the reformat for you. You will lose any data that you don't backup elsewhere in this process. Disk Utility should be found in your Utilities folder in the Applications folder. There are several options for formatting that can't be adequately explained here. If the Help documents for Disk Utility are not enough, there'll be plenty of info at the usual sites, e.g. MacWorld or CNET.

There is commercial software that will shuffle current data around as it repartitions into a single partition without deleting. ( TechTool Pro costs $100 http://www.micromat.com/techtoolpro) BUT YOU ABSOLUTELY SHOULD STILL BACKUP to be safe so you might as well stick with Disk Utility.