We're going to boot your Mac to a special mode called Single User Mode. Then type some commands/texts there which will force your Mac to bring the new account setup screen. So you can setup a brand new admin account for yourself. This might seem a little difficult, but it is actually easy.
Follow the steps below to create a new User Account in Single User Mode.
• Turn off/Shut down your Mac.
• Turn it on and immediately hold down the Command(Apple) and S keys together for 10 seconds to boot into Single User Mode
• When you see the text prompt (Single User Mode), type in the following terminal commands to create a brand new Admininstrator account (hitting return after each line). Also, note down the spaces between the commands, like a space between mount and -, space between -uw and / etc
mount -uw /
shutdown -h now
(If it returns any errors, then you are entering the commands incorrectly. Make sure you enter it correctly)
When you enter last command and press return, the Mac will shutdown. So turn it on, you should be able to create a brand new Administrator account.
If you want to change the password ***** old account, you can do that using the new admin account. On the new account, go to System Preferences > Accounts (or Users & Groups). Unlock the lock there. Then select the old account and click Reset Password. Then reset the password.
After that logout from Apple menu and login to old account. Now you may be asked for keychain thing. So click create new keychain and reset keychain database. Follow the steps below;
Open Keychain Access, which is in the Utilities folder within the Applications folder.
From the Keychain Access menu, choose Preferences.
Click General, then click Reset My Default Keychain.
Authenticate with your account login password (the password ***** reset earlier).
Quit Keychain Access.
If you have any difficulty on following the steps above, please let me know and I will continue to help you.