Definitely do the procedure listed in MrHoffman's reply first.
If that doesn't work, however, the fact that you are running on an older system means you should most likely be able to do the following process to make a new Admin user if you didn't have FileVault (encryption) enabled. With a new Admin user it's easy to reset passwords (but as pointed out, you'll still need to reset your Keychain and lose all your saved passwords):
1) Boot up the mac holding COMMAND S. This will boot you into “single user” (text only) mode.
2) Type the following commands:
mount -uw /
rm /var/db/.applesetupdone
shutdown -r now
If you don't get any error you will have removed the hidden .applesetupdone flag file and this will kick your Mac into the out-of-the-box new Admin account setup!
You well see your “Welcome” mac user screen. No worries! This will not affect anything you have on the Mac. Your regular user account, data, apps etc., will be just fine. This process is just coaxing the Mac to generate a new Admin user. Just give it a name like "Test" or whatever.
You can log into that "Test" account and reset the password for your other real user account(s) in System Preferences > Users & Groups.