what do I need to erase all data on old 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 6, insert a compatible Mac OS X install DVD, 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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Erasing the data on an old computer can be done by following the steps in this support article –

How to erase an Intel-based Mac - Apple Support:

https://support.apple.com/en-us/HT20106