how do I delete everything so I can donate my computer?

Click here and follow the instructions. If the computer originally shipped with Mac OS X 10.6.8 or earlier, when you reach step 5, insert its original disk, restart with the C or Option key held down, use the Disk Utility to erase the internal drive, and install a fresh OS.

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Restart and hold the Command and R keys, from the drop down utilities select Disk Utility, highlight the Macintosh HD, and select Rease. After that completes select Install Mac OS Xto reinstall Mac OS X.

If you did not buy the Mac with your current version of OS X the process is slightly different. So, if you had a different version, what Mac OS X came installed?