Welcome to the forums!
I recommend you do a fresh reinstall of Ubuntu (or another Linux "distribution" like Mint).
This will give you a "clean" system without some stranger's data, settings, and config.
It should take you less than an hour to download, install, and set up Ubuntu. Instructions at ubuntu.com
However if you just want to reset the password and get on with it: http://www.psychocats.net/ubuntu/resetpassword