Don’t have a clue what my Apple ID or password to my MacBook early model OS X El Capitan

I do not have the password email address or any information of who the previous owner was to this MacBook.I found while I was trashing own an apartment where I work . I had ti but a charger and the old thing turned on if would like to erase it completely witch I have erased in the disk Utility pop up but then To try to do anything else it tells me my Apple ID or password is need to continue witch I don’t know !……so is there a disk I can buy to just reinstall it new again and it will work ? I’m not to tech savey neither so simple is better if possible