[ubuntu] How to Format Disk?

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http://ubuntuforums.org – I want to sell my computer and would like to delete all traces of me in it. The easiest solution (at least in theory) should be to format the disk. But how do I do that? I don't have time to learn and navigate a maze of code, so a simple command or a point-and-click solution would be great. I have Ubuntu 12. I assume I only have one disk, but I'm not sure. I have no idea what's actually in the black box under my table. Alternatively, if formatting the disk is too complicated, where exactly are the traces of me that I don't know about? I already deleted all the files/data that I have actively (Hardware)