I can’t remember what my password is to lo into my iMac.

If you do not have an appleid registered to your iMac:

you can boot up recovery mode (hold CMD + R keys when you hear the startup chime, and then keep holding until you see the Apple logo). In recovery mode, press utilities, and then press terminal. Now type in:

resetpassword

With an Apple ID on your Mac:

Reset your password via iCloud.com

Hope this helps!

This time you might also consider writing it down somewhere.