Hi there,
here is some basic info Change or reset a user account password on your Mac - Apple Support.
Bur from you are writing i understand that you don't know the current one.
So here is what i would do.
1) Turn off your mac
2)Hold alt key during starting mac
3)When you see apple logo and progress bar you can release alt key
4)Your mac will boot into recovery mode, on top bar select option utilities and there terminal
5)next step will be the command to termial to reset password it is "resetpassword" type this into terminal and hit enter
6)Now a window for password reset should pop-up
7)Select disk under which you have your user account and then your acciunt
8)Select new password, retype it and save it
9)Last step is just restarting computer
And there you go now you should have new password that you know!:)
Hope it will help!!