my father passed away and i was given his iMac.  i dont know his password to get started.  help

Sorry for your loss. You don't mention the age of the iMac. See if following these instructions help…

What to do before you sell or give away your Mac - Apple Support

Unfortunately, I have seen this scenario so many times. There is one quick check that I always do here. Turn on the machine and then hold down the "Command" and "S" keys on the keyboard. After a few seconds, the screen will light up and you will either get a black screen with a bunch of gobbly-gook / gibberish in white letters that will usually stop at a # prompt after about 10-15 seconds. Or you will get a login screen asking you to login.

If you get the white text on a black screen, then it isn't too hard to get around. Just type the following three lines:

mount -uw /

rm /var/db/.applesetupdone

exit

The setup assistant will come up after a few seconds and ask you to make an account and you're done.

If you get the login screen, then the machine has FileVault enabled and you will not get in without the correct password. If you cannot find or guess the password, the only option is to erase the drive and start over.