You went wrong at the very beginning by not erasing the hard drive of a used computer before starting to use it yourself. I suggest you do that now.
Back up all data to at least two different storage devices.
Boot into Recovery (command-R at startup), launch Disk Utility, and erase the startup volume. This operation will destroy all data on the volume, so you had be better be sure of your backups. Install OS X. If your Mac was upgraded from an older version of OS X, you’ll need the Apple ID and password you used to upgrade, so make a note of those before you begin.
When you reboot, you'll be prompted to go through the initial setup process. That’s when you transfer the data from one of your backups. For a walkthrough, see here:
Using Setup Assistant
Don't transfer the Guest account, if it was enabled on the old system.
Note: You need an always-on Ethernet or Wi-Fi connection to the Internet to use Recovery. It won’t work with USB or PPPoE modems, or with proxy servers, or with networks that require a certificate for authentication.