Questions About Security

I am having trouble creating a User ID or password.
Can my password be the same as my User ID?
Does my password ever expire?
I forgot my password. What do I need to do?
If I forget my password and change it but remember it later and want to change it back to the original password can I?
I tried to register an account on the web site and received the message "user not found." What should I do now?