Glad to have you on the dark side. :)
As far as any tutorials on how to uninstall Win 8/7 so you can bring Ubuntu on board, there really is no need to. As soon as you install Ubuntu, and by extension, the ext4 filesystem, it will completely dissolve the old Win 8/7 filesystem and take up all the disk space (if that's what you're wanting, there's more options than that though.).
However, if you REALLY want to "uninstall" (again, totally unnecessary) Win 7 or 8 before you install Ubuntu, the easiest course of action would be to take an old Windows CD such as XP or ME (if you happen to have one laying around); boot into it, then follow the instructions and delete the Windows partition. After that, just pop in Ubuntu, install, and you're good!
Also, as a side note, you may not want to kill off Windows completely. You may want a dual boot system so that if you're having a problem with Linux, you can use Windows in the mean time. But, that's totally up to you, it's just something I wish I had done when I switched. If you have any questions, please let me know, I'd be glad to help.