how do i find out my iMac password

If you forgot it, you can't find out what it is. You need to reset it. Which password are you referring to?

What version of OS X are you running? 10.6, 10.7?

>about this mac.

here are directions on how to reset it: http://osxdaily.com/2011/08/24/reset-mac-os-x-10-7-lion-password/

Pamfromvic wrote:

If I shut down the computer (rather then just put it to sleep) do I need the password to start up again?

Well, that depends on how your settings are. But since you don't have your administrator password, you can't make changes anyway. In the future when you reset the password and regain access you can find these settings in >system preferences>security and privacy general tab.

Someone has told me I should be shutting it down when I'm not using it.

No, that is not necessary at all. Putting it to sleep is just fine when it is not in use. Restarting it once a month is ok to clear out the caches, but everytime you are not using it-that is not necessary.

Pamfromvic wrote:

If I shut down the computer (rather then just put it to sleep) do I need the password to start up again?

Only if the "Require password .......... after sleep or screen saver "Check Box" is ticked.

Once you have reset the admin password so you know what it is, you can set the machine to boot with automatic login for a specific account, meaning you would not need to enter that account's password at boot. This setting is in System Preferences > Users & Groups, then clicking the Login Options item.

Caution - by setting login to automatic, anybody can use your machine when you're not there. I use automatic login because it's convenient for me, and there is no chance of anyone else using my computers. This is not the case for everybody.

Welcome.