How do I erase an ibook g4 so that I can donate it?

Go to System Preferences>Accounts. If necessary click the Lock button and enter the admin password. Click the plus button at the bottom of the left-hand column and create a new user account - give it a name, a short name and a password. Check the box to 'Allow user to administrate this computer'. Now you have an empty admin account (you must have at least one).

Then select your current user account and click the '-' (minus) button next to the plus button to delete the account. Note that data could be retrieved by a retrieval program though at this age it's unlikely anyone would still have one. If you have any sensitive information you should drag it to the Trash first, then 'Secure empty Trash'.

Anyone you pass it on to should be aware that without the install disks they will be unable to recover from serious problems unless they can find a Tiger install disk on eBay or similar. Even if they can this sort of thing tends to be rare and thus expensive.

You should have the original installation disk. Insert it and start up holding the 'C' key down to start from it. When it's loaded (it will be quite slow) choose 'Disk Utilities' from the 'Utlities' menu, select the hard disk and erase it. There should be a 'secure erase' option of some sort and you should choose that - the most basic erase will erase only the Directory.

Then you can install the system and any bundled applications to restore the Mac to its out-of-the-box condition. However Tiger is very limited in what it can do these days; many websites won't work properly. However basic email and simple word processing should be OK.