Do you mean the zip code on your Bank of America monthly statement?
That should be the same zip code as your home address, so you should be able to find it on any mail addressed to you, especially utility bills and other services.
If you don't even know your own zip code though, I'd be slightly worried...
So maybe what you're really asking is what is the zip code of your local Bank of America branch?
Since you haven't disclosed any information about your geographic location, all I can suggest is that you go to the Bank of America website and use the branch locator service to find your local branch.
The zip code should be listed on the branches page from there. Good luck!