How do I reset my password on my apple computer?

Check out this article, it gives many methods to reset the password

https://support.apple.com/en-us/HT202860

If you still can't log in with your password, contact Apple Support for help.

Last time I had this, I had to reset the computer entirely but only do this for last resort, you'll lose everything.

It varies with when the computer was manufactured so, to get a specific answer, you need to tell us the model and sub-model of your computer and what version of macOS it is running. Please also tell us which password? Your Apple ID, the admin password?

Otherwise, we are guessing, and we are not very good guessers!

Also, through Apple's non-intuitive forum labeling, you found the forum section for iMacs made before 2006. Is your computer that old?