how do I clean out my very old (2011) macbook

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 its original 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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If the laptop uses a hard drive, then I highly recommend you first enable Filevault and let the encryption process complete before attempting to erase the drive. Apple's documentation that @Niel linked falsely assumes a user has their hard drive encrypted with Filevault or their Mac is using an Apple SSD.

If you are using a hard drive, then a simple erase will still leave your personal data behind where a simple data recovery app could access those files. First enabling Filevault on a hard drive securely encrypts the data so when you perform a simple erase of the whole physical drive you destroy the encryption keys so that the data can never be accessed. If you don't first enable Filevault and your laptop uses a hard drive, then you will need to do the more difficult option of performing a secure erase which writes a single pass of zeroes to the entire drive.

Note: There are no 2011 MacBooks, 2006-2010 and 2015-2017.