root access - Use stock recovery to install superuser?

I am trying to use the stock recovery to install superuser.zip but it give me an error: E: signature verification failed

So I cannot root it by the superuser.zip

I Googled and someone said solution is to install CWM. However, when I try to install CWM, it require superuser privilege?!

Now I get into the loop CWM need superuser -> install superuser need CWM -> ...