New to Mac, reinstalling OS if that's you call it

I just bought a slightly used Mac book and want to know the procedure to wipe the previous users data and start from scratch. I just reinstalled the OS but it retained the previous users data

Thanks for any suggestions!

When loading off the supplied instal disk before you click on final install button you get the choice to customise - leave off languages etc and Options - which will have one for erase and clean install.

Thanks for the quick response, however I wasn't able to see one for erase and clean install.

I did this:
1. Enter single user mode(command + S after restart). Once in single user mode, check the filesystem and mount the hard drive.
fsck -y
mount -uaw /

2. All we need to do now is remove a flag file from the system
rm -rf /var/db/.AppleSetupDone

3. Now reboot
reboot

I am entering my user data now, thanks.

Boot from the install disc and when you select the drive you're installing it on, there should be an Options button - choose "erase and install". You might also consider disabling extra languages and unneeded printer drivers (the next screen will have a "customize" button).

Well I didn't see any option to "erase and install", but this is the first time I have ever used a Mac so it's quite possible I just didn't see it. So at this point I have reinstalled the OS, without the erase and install option, and reinstalled the apps. I used the method I noted above to re-initiate the setup process and I now have my user data on the machine. Is there any reason for me to go back and attempt the erase and install at this point? If the old user data is still present wouldn't that space now be made possible to be written over?